It really depends on your local conditions.
the 4.0's are what... 118 underfoot? I'm guessing these are only going to be used as a super-deep pow day tool, and you arent worried about negligible weight differences. This makes it easy.
If you have heavier, wetter snow where you are, get 100% Nylon. If you have lighter, colder snow where you are, get a mohair mix skin. If you just want better traction overall, go 100% Nylon.
All the skin makers are putting pretty decent stuff on the market. I have been using G3s and BD skins for years and both are pretty great offerings. Ive also used Pomocas and theyre great too. You kinda cant go wrong my friend.