For me, iStripper animations in 4K are MUCH better (I was Premium some years ago). Images are more crispy, more definited, you see more details. But among other members, some is not able to see the difference... I think only way to decide if it suits you, is to check with your own eyes.
If you have a 4K display, I think getting 4K girls is a no brainer. Generally quite a bit crisper. But if you don't have a 4K display, it won't make much of a difference. You shouldn't see a bitrate difference unless you are running quite an old graphics card and /or like to run with multiple girls on the screen at once.
The files are downloaded to your device, and the 4k are bigger than the 3k ones. If you actually have a 4k monitor the better resolution is ... uh ... better.
I would say that resolution matters on when zooming in to the maximum size and when you have a monitor larger than 27 inches. The greater the frame, the greater the potential of sophisticated detail making a difference within the expanded video or image, since when the screen gets larger we tend to focus more on the smaller details that become larger on our desktop viewing angle. Just my two cents, hope this helps! 👍