|G4 is the processor not the model name of your computer. Do you have a G4 eMac, iMac, PowerMac, iBook, PowerBook, etc....|
You most likely installed incompatible ram and the the pram was changed.
Reset your PRAM, power the computer on and hold the Command, Option, P, R keys until the computer pongs twice.
If you can get into open firmware, reset your NVRAM. Hold Command, Option, O, F keys until you get into the open firmware screen
So, give that a try.