V4 textures should work fine with Genesis 1, you’ll just need to make sure that the UV map is set to V4 though I think that’s the default. In fact if you look at them a lot of the V5 characters are using the V4 UV map.
To use a V4 map on Genesis 2 or V6 you’ll need the V4 to Genesis 2 Female converter and you’ll need to make sure your V4 materials are saved as a .duf file.
I may be biased by being the creator of the product ‘Corrected V4 UV for Genesis’ but one can not honestly state that the original V4 textures works ‘fine’ with Genesis unless one point out the obvious.
V4 textures without any make-up look decent on Genesis. The most obvious problems with the body (which my product does not correct) are the knees which moved sideways because the way the uv seam moved from the V4 to Genesis mesh in that area. The elbows also suffer from from positioning errors. Dependent on the used texture this may or may not be a problem.
What’s with the face then? With make-up, the first thing one notice is the outer corners of the eyes, then the top left contour of the lip, then the left nostril, then the rest of the face…
The V4 for Genesis 2 Basic Female face uv mapping works fine as it is created.