It is not so hard but takes a lot of work. You want to convert gen8 skins to v4.

Both mods have different starting poses. I would set g8 in a t- pose. Export to zbrush and also v4 to zbrush. Make sure the poses line up as best ad you can.

Now you have to append g8 to the v4 model. Set it on transparant. Morph gen 8 as close as you can to v4. The better the morph the better the texture will look.

If you are done. Load the textures on Gen8 via polypaint. Now click on uvs from polygroups and do the same for v4. Subdivide v4 as much as you can. Then hit project all. Delete the lower resolutions. And make every bodypart invisible. Set the uv resolution and export the texture. Do this with every other bodypart, head, arms etc.

Nb, make sure you delete the inside mouth areas and lashes and eyes.

Remember it is not perfect and you will probably have to clean up some things. But this is the easiest way.

The higher your subd the higher the textures.