Why is the signal-power-signal-ground 4 layer PCB stack frowned upon?
If SMT components are mounted on both sides, you would rip up the outer power/ground plane in a S-P-S-P (Signal - power/gnd Plane - Signal - power/gnd Plane) stack up in order to place solder pads and place traces between these pads and vias.
In order to reach or to leave the signal plane that is sandwitched between power/ground planes, you **have to* use via's. This will detoriate the return path.
Alhough this problem can also exist with a S-P-P-S stack up, you may find ways to avoid using vias by routing the signals subjected to this issue first.