In a 4-layer PCB, operating at a 8-16 MHz clock speed, should there be any copper pour on the top and bottom signal layers?
All copper on the board should be consciously designed. 

I am against copper pours on principle, because they seem to cause more problems than they solve - increased stray capacitance, confusion about where return currents are flowing, unintentional coupling between signals, or creation of an antenna. It can complicate debug and modification as well.

They seem to lead to a lazy mindset, where the designer thinks 'well there's lots of copper on the board, surely the grounding will be OK now!' A copper pour is unnecessary, and insufficient to achieve a good board.

There are good reasons to use a copper pour, thermal conductivity, reducing the amount of etch solution used, improving thickness tolerance, reduction of warping, beefing up of tracks, but 'should' isn't one of them.