PCB Layer stack up related query
For controlled impedance and low noise you'd like the signal traces to run over a continuous ground plane. In theory the power plane might act in a similar manner, however with many boards it is desirable to split the power plane into different regions, whereas the ground plane is usually left intact (at least up to isolation barriers). 

You do not want to run signal traces to run over a break in an AC grounded plane because that can cause problems with EMI as well as signal integrity. 

If you have many parts on both sides you may wish the penultimate bottom layer to also be a ground plane.