Since cascade 0 is on that that starts from camera position and affects most notticable details its good idea to give most of resolution and distance to it and for latest cascade it would be stupid enough to give much of resolution and distance since its far enough.

Also distance scale of 1.0 seems to be almost equal to 10 meters. Increasing Cascade0 to 10.0 will render a lot of detailed shadows for area 0-100 metters from camer. setting distance scale 10.0 to cascade1 will render cascade1 at distances 100-200 meters, cascade2 = 10.0 will render cascade2 at 300-400 meters, and cascade3 = 10 fill render cascade 3 at distance 400-500 metters.

Since resolution of texture will remain the same, each pixel of shadow will need to cover bigger are, which will lead to decrease of shadow quality.

The only way to compensate it is to rise cascadeshadowresolution to 8192 (it seems to be maximum game accepts now with tweaks, but my card GTX 680 allows texture sizes up to 16k so i hope CDPR will remove texture size cap for everything and will give me accces to 16k shadow an other resolutions, which i can afford with 4gb vram.
There is a way to rise texture resolution cap, actually i did it by rising 2048 limit to 8192 for everything in startup.bundle, but i cant set 16k since it needs extra symbols added to each line, and with different size i would not be able to re-import it with QuickBMS, so i hope somebody will create proper unpacker\packer for bundle files that will allow me to import file of any size so i can rise limit of texture resolution up to 16k.

Settings ofr @Asmodean778 will give next results:
Cascade0 renders at distance 0-15 meters
Cascade1 renders at distance 15-45 meters
Cascade2 renders at distance 45-70 meters
Cascade3 renders at distance 70-90 meters

And with his settings Cascade1,2,3 gets more space in final shadow texture and reducing details of cascade0 which is bad.

Logic for cascade shadows should always be like this

Cascade0>Cascade1>Cascade2>Cascade3

So best minimal settings to start with would be

CascadeShadowDistanceScale0=4.0
CascadeShadowDistanceScale1=3.0
CascadeShadowDistanceScale2=2.0
CascadeShadowDistanceScale3=1.0

This will span shadow map across 10 meters like this
0=0-40 meters >1=40-70 meters>2=70-90 meters>3 = 90-100 meters.
Resolution will change relative to cascade with lowest one, so since cascade3 has 1.0 it will be out start point
C0=C3*4 > C1=C3*3 > C2=C3*2 > C3=C3.
Im not exactly sure how each cascade mapped to texture, they could be for example fixed in Width and have scalable heigh and aligned in one line, or they could be a square shaped and dynamically change position in texture grid depending on scalling, but i bet on 1 fixed dimension and 1 scallable.

In this is true, than with resolution of cascade texture 2048x2048 we will have to solve this (X)+(2X)+(3X)+(4X)=2048 to get idea what resolution each cascade. has. SO with integer values it would be easy and simple 10X=2048, which means x=2048/10=204.8 (most likely converted to lowest closes Power of two integer)
So cascade3 most likely has 204x2048 resolution in my example would be:
818x2048 > 614x2048 > 408x2048 > 204x2048

Seems reasonable decrease or resolution at distant cascades, right?

Now lets do some math to understand why settings of asmodean was bad
1.5X+3.0X+2.5x+2.0X=2048
9X=2048
x=~226
So each cascade woult be scale*x and resolutions would be
~338x2048< ~678x2048 > ~564x2048 > 452x2048

So closes shadows gets less detailed resolution, than most distant? This is why this is no right!

So lets do another example to understand better how shadows works in witcher and how you need to calculate settings for them for best look and expirience:

For example you want shadow to cover distance of 200 meters and want detailed shadows to stay detailed at maximum possible distance, lets say 100 meters

than we need to set it like this for first 100 meter:
CascadeShadowDistanceScale0=10.0

and we have to split remaining 100 meters which is 10.0 between cascades 1-3, and its good idea to keep integer values here (2 and 3 instead of 2.5 and 2.5 ) and even better idea to keep power of two integers (6+2+2 instead of 5+3+2)
it would be
CascadeShadowDistanceScale1=6.0
CascadeShadowDistanceScale2=2.0
CascadeShadowDistanceScale3=2.0

So each cascade will cover area:
0-100 meters > 100-160 meters>160-180 meters > 180 - 200 meters

Makes sense, isnt it?

Now what about resolution?

X = CascadeshadowResolution * (Scale0+Scale1+Scale2+Scale3)
Cascade[N]resolution = X * Scale[N]

I suggest to not count X based on cascade shadow resolution but count final resolution based on some pattern X.
Depending on you system performance i would take these as base x, since final shadow resolution is what gives most of performance hit:

16<32<64<128<256<384<512

So lets start with 16 for very slow machines, cascade resolutions would be

16*10=160 > 16*6=96 > 16*2=32 = 16*2 = 32
and cascade shadow texture would be
160 + 96 + 32 + 32 = 320, which is way too low to look good. so lets try better X.

Lets start with 128

Cascade resolutions would be
128*10=1280 > 128*6=768 > 128*2=256 = 128*2=256
and full texture resolution is
1280+768+256+256 = 2560

Now thats a good sweet spot between quality and performance, but lets not forget that cascade 0 should cover 100 meters, so 256 or even 512 are much better values, lets take a look at 256 as X:

Cascade resolutions:
256*10=2560 >266*6 = 1536 > 256*2 = 512 = 256*2 = 512 - now that an improvement we got complete 2560 from previous example just for Cascade 0 alone and minimal size of cascade is 512!

2560+1536+512+512 = 5*120 that good enough resolution, but we still have space up to 8096 (and with proper bundle editor possibly up to 16k ) so lets try 384

384*10=3840 > 384*6= 2304 > 384*2=768 = 384*2=768

3840+2304+768+768 = 7*680 - almost perfect, we almost reached 8096 with good power of two resolutions for each cascade, but lest get a little more

for example we can add 32 or 64 to 384

384+32=416

416*10=4160 > 416*6 = 2496 > 416 * 2 = 832 = 416*2 = 832

8320
Oops. we got 8*320 a little more than budget we have, we can optimise it by lowering render distance to 180 and taking 2.0x out of any cascade like this

CascadeShadowDistanceScale0=10.0
CascadeShadowDistanceScale1=6.0
CascadeShadowDistanceScale2=2.0
CascadeShadowDistanceScale3=1.0

416*10=4160 > 416*6 = 2496 > 416 * 2 = 832 = 416*1 =416
4160+1664+832+416 = 7*904

CascadeShadowmapSize=7*904

Perfect! We are under 8192!

Now lets optimize a little more to keep good balance between quallity and performance by reducing cascade shadow map size and render distance of shadow map but yet keeping good looking shadows, lets say we want cascade 0 to be 4096, so lets take X=512 and set cascades like this

CascadeShadowDistanceScale0=8.0
CascadeShadowDistanceScale1=4.0
CascadeShadowDistanceScale2=2.0
CascadeShadowDistanceScale3=1.0

We will have cascades rendered like this
C0 = 0-80 meters > C1 = 80-120 meters > C2 = 120-140 meters > C3 = 140-150 meters

And resolutions of cascades would be

C0=512*8=4096 > C1=512*4=2048 > C2=512*2=1024 > C3=512*1=512 - seems like perfect pattern, just like mip maps for textures!

4096+2048+1024+512=7*680

CascadeShadowmapSize=7 680

Seems like perfect mix between quality of each cascade, render distance and performance.

But we have slower machines and can optimize this even further. Lets not change distances and scaling anymore but just take another base X=256.

C0=256*8=2048> C1=256*4=1024> C2=256*2=512> C3=256*1=256

2048+1024+512+256=3*840

CascadeShadowmapSize=3*840

Now thats a good value for perfromance.

But if you have a weaker machine you can reduce resolution twice to

CascadeShadowmapSize=1*920 (its even performs a little better than default high shadow settings with 2048 resolution)

and will have cascade sizes
1024>512>256>128

But if thats still a performance hit because of distance, you can try to reduce each distance scale by 2 (except for last cascade) to :
CascadeShadowDistanceScale0=5.0
CascadeShadowDistanceScale1=3.0
CascadeShadowDistanceScale2=1.0
CascadeShadowDistanceScale3=1.0

This will render shadows like this

C0=0-50 meters>C1=50-80 meters>C2=80-90 meters >C3=90-100 meters.
With such settings you will have shadow render distance double comparing to default (1+1+1.5+1.5=5.0 aka 50 meters) and yet texture resolution and distance proportions would be better re-distributed for quality since you will have half of distance to cascade 0 alone.

You may wonder how come that default values are so different - i will tell you then that i wonder how game which looks and have less advanced tech than Crysis in 2007 can run well on hardware that gives 120+ fps in Crysis. But answer is simple, im not sure who responsible for optimization in CDPR but those people really did a bad job, they cut graphics a lot and yet failed to optimize game, it seems to me like people who was optimizing game like this was not even well aware of how their own engine works and to find sweet spot between graphics and performance, its seems like if level designers, artist, and even testers optimized this game in a rush after someone from above told them "optimize your assets" and forgot to mention that he is lazy to incapable to optimize red engine by art of coding, and it feels like some core programmer of red engine quit company and there was nobody to guide them and tell them they doing something really stupid.

So default settings for shadow cascade distances makes no sense at all, somebody who really is not aware of how red engine and shadow cascade works set these settings or probably just altered a little default stub values of 1.0 set for everything and 1.5 for Scale2 and 3 resulted just by observation of that someone that it increasing shadow distance a little and thats about it.

And everyone who tweaking game by keeping values close to defaults is doing mistake.

So to summarize, forget about default cascade shadow settings! They are joke!

Keep these rules in mind:
1) Cascade 0 should have at least 2x bigger resolution and distance than any of following cascades. Having distant cascades with resolution higher than close cascade is waste of resources and quality for nothing ant total nonsense!

2) Its good to keep cascade scale values ad integers and power of too, except for value of 1, which is not power of two but good values:

3) Final render distance is based on sum of all cascade values, value of 1 equal to about 10 in-game meters of distance.

5) All cascades shares same shadow map texture, increasing its resolution decrease resolution of other cascades

6) With great resolution comes great responsibility greater number of small objects casting shadow.

7) CascadeDistanceScale controls both render distance if meters (multiplied by 10) and proportional size of cascade inside shadow map texture. So this means that increasing distance of cascade 1-3 will make their resolution higer and will reduce resolution of cascade 0, and this will lead to some objects loosing shadows and shadows getting low res jagged and blurred look at close distance.

Cascade distance and resolution should be distributed from MAX to MIN, period,
Settings like 1 < 2 < 3 <4 will makes shadows worse and will waste performance and quality for no good reason !
settings like 4 > 3 > 2 > 1 will make shadows look better with almost same performance cost!
But settings like 8 > 4 > 2 > 1 will make everything even more better.
9) Cascade Shadow resolution should be increased to make shadows look good, you can calculate final optimal resolution for it by using formulas and logic explained about, but shorter version would be:

CascadeShadowmapSize=(CascadeShadowDistanceScale0* +CascadeShadowDistanceScale1+CascadeShadowDistance Scale2+CascadeShadowDistanceScale3)*X

where X is value that you can devde by 4 many times (not sure how to tell this right in english, since its not my native lang) and get integers in the end like this
(256\4=64, 64\4\=16, 16\4=4). Minimal good value for X is 128. You can go further by add 16, 32, 64, 128 to it until you will find sweet spot between quality and and performance.

Recommended value for good quality is 512, for medium is 256, and for low is 128. Try not to exceed limits of 4096 by default, and 8192 or 16k if you have modded resolution cap (see below)

10) Best way to set your shadow cascade sizes and distances is to start from last one which is cascade3 (4 actually but you know, in weird programming world everything starts from 0, lol) as base value, and go to previous cascade by multiplying it by 2, and repeat it until you will get to cascade 0.
So this means if your cascade3 is 1 your cascade2 should be 2, cascade1 should be 4 and, cascade0 should be 8! Alternative way for slower machines would be adding 1 to each cascade by going from distance to closes one, so it would be cascade3=1, cascade2=2, cascade1=3, cascade0=4, you can also add 2 instead of 1 and get values like this C3=1 C2=3 C1=5 C0=7 .

11) Some level designers took shortcuts or messed things up during rushed optimisation of game, so some objects does not cast shadows in cascade 0, and only do in 1, you can do nothing GOOD about it now, patched assets or engine needed to force all objects to cast shadow in cascade0 range. Only with official patch, unofficial memory \ or hex assets hack, or redkit this can be done

12) to uncap resolution of shadows and other textures you have to unpack Conten0\Startup.bundle with QuickBMS + Witcher3.bms go to
engine\textures
open textures.xml in good editor like Notepad++ or PSpad and change anything like 1024-2048-4096 to 8192, but until proper bundle editor releaased dont touch 512 and dont try to set 16k since this will change size of file and quick bms would not properly re-import it and game would crash.

IF you are lazy to do your own calculations and just want settings you can play with or start tweaking, this is default base with optimal performance and quality on high end machines everyone (who plays with uber settings) should stick to: