I have developed a 4 layer PCB and devoted the middle two layers to GND and VCC. What should be done with the outer two layers? Should I create planes on those as well and set them as GND layers as well?
A few decades ago I worked on 8 layer boards. For those boards we did center layers as power and ground and all other layers were for signals only. In those days we were doing all digital at frequencies to about 20 MHz. I would expect that just using the outer layers for signals would be fine. You might on critical signals add guard traces or planes.
- Chip
Some people leave them, others add copper pours where there is space & tie it through to the GND plane with vias.
In general - the more gnd copper the better, trying to balance out each layer so it has a similar amount of copper all over - this prevents warping of the board when going through hot processes.