In a 4-layer board the inner layers provide the return path for signal traces on the
surface layers - the surface trace and buried ground/supply layer form a transmission line,
so its vital to both connect the inner power layer to something, and that that something
is either ground or decoupled to ground in many places. (Assuming you have fast logic signals).
However here it sounds like you might have no signal traces on that side so it won't
matter (assuming of course that the spare inner layer is that side).