You probably have mixed DDR400/PC3200 memory that was in the G5 originally with DDR333/PC2700 memory from the G4.
You can try installing pairs of identical memory in each set of slots starting from the inside out, and try to continue to use the memory from the G4, but it just may not want to run with that.
Generally, I use 1GB strips of PC3200(DDR400) memory, but that can be harder to find, so 512MB strips like the four you started with would work as well.

G4s and G5s work well with either DDR333 or DDR400, but I'm not sure about mixing the two (could be problematic).

Anyway, first step, remove the G4 memory, under MorphOS you can't use more than 2GB anyway (unless Andreas corrects me on this as I haven't checked lately).

Good luck.

[ Edited by Jim 05.10.2017 - 14:59 ]