Multiple ground planes in a 4-layer PCB

[Piotr]

@Will_Zhao

do I need to add some ground copper pours so that the 2 middle planes can be of similar size?

I’m not an expert. I have never designed 4 layer PCB.
I have read that if you have the same copper pours at adjacent layers you form a kind of waveguide with open ends that allows for standing wave what is not good. If remember well it was written about regular rectangles what is not a case here. There were written that at one layer copper pour should be smaller.
I probably read it in one of articles I have linked here:

Decoupling capacitor via placement

I didn’t read that book but I suppose you can replace it with: I think it is enough for most pcb designers.

and here:

Decoupling capacitor via placement

About 2004 I found an interesting serie of EMC articles by Keith Armstrong (for many years I didn’t know if Keith is a woman or men name). Later they became no accessible, but when I tried to find them few years ago I found them once more: http://www.compliance-club.com/pdf/DesignTechniquesPart1.pdf http://www.compliance-club.com/pdf/DesignTechniquesPart2.pdf http://www.compliance-club.com/pdf/DesignTechniquesPart3.pdf http://www.compliance-club.com/pdf/DesignTechniquesPart4.pdf http://www…