What is the proper placement of VCC and GND copper pours in 4-layer PCB?
5MHz is not an issue. Just use the two inner layers for VCC and GND. You will still need bypass capacitors, of course, as close to the VCC pad as you can put them. A via to GND should be fine. 

When I do four-layer boards, I either do not bother with copper pours on the outer layers, or I just do GND pours. This is just cargo-culting, I do not have a good argument for doing it either way, except that it looks better with a copper pour even under the solder mask :)

I do not normally do very high speed stuff requiring transmission-line impedance calculations or worrying about equal track lengths and so on, so don't take my word as received wisdom.

It would be great if one of the more knowledgeable members of the board could point us to a good reference for what constitutes best practices for high-speed design (for various values of "high speed") in the context of amateur work.