4 layer PCB- fill free space on Top and Bottom
I try to keep it short: 

First of all, for a 4 layer PCB, try to put the middle GND plane near to the plane carrying most sensitive or "fast" components in order to shorten the critical vias from the components to this middle plane. 

Whether or not you want to fill TOP/BOTTOM free space with GND depends on how segmented this GND fills will be and how good you can stitch them to the middle (solid) GND plain. If you fill these free spaces, then also try to provide a lot of stitching vias, especially on the edges of the board (building a "cage"). Because if you have a big TOP/BOTTOM filling with only one stitching via to the middle (solid) plane, this acts like a "patch" type of antenna.   

For a 4 layer PCB in general, I would not recommend TOP/BOTTOM fills. 

Keep in mind this general rule: You must provide the lowest possible resistance path  for current to flow from each IC's VCC to GND pin.