Check your display cable and make sure that it is secure. The issue you describe is a:
a.) A graphic card failure.
b.) A CPU failure.
c.) A loose display or LVDS cable.
You will want to check all three of these issues. Do you hear a startup chime? If you don't then it is probably "a" or "c' and could even be "b." Whenever I turn a computer on and it is not working properly, I go back and check for loose cables and so far, that has fixed 100% of my issues. The way the g4 is laid out it is very easy to loosen a cable even if you never touched it. I would try this first then try a few other troubleshooting procedures. Tell us what you find so another community member or myself can provide you with any other assistance.
Hope this Helps