https://www.1addicts.com/forums/show....php?t=1208743

D4 is a little heavier than D6, so probably a little better in warm climates
I am in a pretty warm climate but I've used MTL and D4 and I think I like the thicker MTL better. With the thinner D4 the transmission doesn't seem as smooth. Both work fine, however.

I mention this because I think the same may be true for D4 versus D6 in a cold climate. D6 will feel a bit more mechanical and shift better before the transmission warms up. D4 will feel a bit smoother but be a little stiff before the transmission warms up. Depends a bit on what you like.