Thanks man!4mm should be fine from what I recall, if that's all you're doing. I did the foil panels, MLV and foam.
If I could go back and redo this, I would just use the all in one products like this:
SoundSkins Pro Plus | 64.5 sq ft Full Car Sound Deadening
Sound deaden the whole car with one roll. SoundSkins Pro Plus, 64.5 sq ft of 3-layer butyl, foil and foam for doors, floor, trunk and roof. 4.9-star rated.soundskinsglobal.com
I spent more $$ and WAAAAAAY more time to get a result that I bet is similar.