Are multiple groundplanes a good idea on a 4-layer pcb?
As a matter of course, I always fill all unused areas with ground - just make sure you scatter vias around so that all the layers are well connected to each other, so that they essentially look like part of the main ground plane.  DON'T fill with ground pours but only have them connected to the main ground plane at one point.

This gives you ground plane closer to signals and power in many areas of the board.

Also, as a rule of thumb, I'd put the ground layer next to the VCC layer.  But, with most designs, you can get away with a heck of a lot (unless you have high speed digital, RF, or analog signals), and often it's not worth worrying about it too much.