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:
The SoundSkins Pro Plus Workshop Kit is an extra large roll of our SoundSkins Pro designed to improve your cars stereo performance and reduce outside road noise in a single material. A SoundSkins Pro Plus roll covers 64.5 sq ft and spans 29.5" x 315" in overall sheet size.
soundskinsglobal.com
I spent more $$ and WAAAAAAY more time to get a result that I bet is similar.
