Here are a few pictures showing a cross section view of actual 1.6mm FR4 4-layer and 6-layer PCB's.
Left: 4 Layer, Right: 6 Layer
Same boards cut at a diagonal to show better detail on layer distances
I found some videos of how they produce multilayer PCB's courtesy of Eurocircuits. Link for videos can be found here.
(I have no affiliation with Eurocircuits, just sharing their interesting educational videos on PCB production)
Page updated and copyright R.Quan © 04 Oct 2011.