Homebrew PCBs

One thing I wasn't clear about after reading the excellent descriptions
is exactly how the 'sandwich' is made up.   I had expected that for a
4-layer board one would use two double-sided boards with an insulation
layer between -- but it sounds as though in fact one uses a single
two-sided board (for the two inner layers) and two single-sided
boards for the outer layers?   Is there a particular advantage in
this?

M Cowlishaw