First of all i dont really know how they work. When someone says 4x100. I have a 05 GTI its a 5 lug. Im kinda intrested in buying aftermarket rims but i plan on going to a BMW after im done with this 100% positve i love my VW but i love BMWs more. My question is if i buy aftermarket rims for my VW will they fit say a e36 or e46 in the future(most likely a e36 328 or M3) Thanks for the help guys.

No, I believe they have similar off-sets, but the lug pattern is different.

The BMW has a different bolt pattern and center bore. It won't work.