to reset pram hold the command (the key with the Apple on it)+option+P+R keys as soon as you hear the start sound and keep them held until you hear the start sound 3 - 4 more times. note that you have to hold the listed keys down at the same time
to reset nvram hold the command ( same key as listed above)+option+o+f as soon as you hear the start sound and keep them held until you see a command line. if you dont see it, you can still do it blind
type the following
reset-nvram (press enter/return)
reset-all (press enter/return)
after these commands are entered the machine will restart itself.
does this monitor use a VGA connector or a ADC (Apple Display Connector). it probably would be good to ask you what model of G4 this is, cause there is quite a few different models. some models have a vga output, some have a VGA and ADC output.
EDIT
additional info
if memory serves, the 20" Apple cinema display (LCD) hooks to the ADC output on the video card. and only video cards with 32mb or more vram will drive this monitor, meaning a Rage 128 pro will not be enough to drive this monitor, the video card will have to be a Nvidia Geforce2 MX (or better) or a Radeon 7500 (or better) or a Powerbook G4 with a DVI port using a DVI to ADC connector, also i think it requires OS X 10.2 or better.
if you have the Rage 128 pro it will drive the 15" Apple cinema display (LCD) just fine, but not the 20"