first stop - could be the internal battery especially if you are getting power to the computer but not getting through to any sort of boot up screen
You can buy replacement batteries from Maplins (UK) or Radio shack (USA)
Costs peanuts and might be the problem, also check that ram modules are all seated properly, but as its a G4, a dead internal battery might well be the cause.
http://www.powerbookmedic.com/Take-Apart-Repair-Manuals-p-1-c-258.html
This my interest you:
1 beep = no RAM installed
2 beeps = incompatible RAM types
3 beeps = no good banks
4 beeps = no good boot images in the boot ROM (and/or bad sys config block)
5 beeps = processor is not usable
Did you intall any RAM?