4 layer stackup, inner layers GND & PWR
If your decoupling is good then there's little difference in principle between using a ground plane and using a power plane for screening.  In a typical stack you'll have the top two layers close together, a relatively large gap in between and then the bottom two layers close together, so placing a ground/power planes on the internal layers will be effective.  For SMT it's wise to track signals on the component side in preference, to minimise the number of vias.  Using a split plane (power and ground on the same layer) is quite possible, but it necessarily fragments the planes and so makes them less effective.  Apart from that, there's no problem mixing potentials on the same layer.