I am trying to get a used G4 working. When it's plugged in and you push the power button, the light will come on only while you have your finger on the button. Nothing else, just the light and only while the finger is pushing the button. Is this a power supply issue? Does anyone know what is causing this problem?
400mhz
1mb cache
20gb hard drive
512mb ram
Thanks.

It is possible that your G4 just needs a PMU (or CUDA) reset, rather than a new power supply. First, ensure that the logic board battery has good voltage; if not, replace it (install it with the correct orientation) before doing the PMU reset. The battery should be from 3.3 to 3.7 Volts, or it is too low. There are several flavors of G4, and I don't know which one you have, but the exact location of the PMU button on the logic board can vary a bit. If you do not know how to do the PMU reset, I suggest reading the following webpage, as it shows the location of the PMU button for several G4 models, in addition to giving the PMU reset procedure:
http://support.apple.com/kb/HT1939

same thing happened to me on a used G4
(did you buy it from an online reseller in uk by any chance)?

light came on just for a second a few times,then powered up a few times, then there was just no power at all,I tried resetting CPU but i think it may be sign of a failing power supply

Thanks, I printed off your post and link and had much new hope but I tried it and it didn't work. Does that mean the power supply is bad?
Here's my situation:I have had a G4 in my garage for 6 months with no power supply at all (not even a bad one, none at all). The guy that sold it to me for 20 bucks said "all it needs is a power supply" so I figure it will be a good project and I will come across one someday. Then a few days ago someone put a G4 tower on craigslist for $20 that is identical to my tower only it has a power supply. He said that it keeps coming up with a software error and I figure it's an easy fix. Especially since I have basically a twin to this tower. Get it home and I get this problem. If you think it could be something else I can put this power supply in my other G4 and see what happens, otherwise I may have 2 towers that both need a power supply.

No, this happened in Omaha, NE. What did you end up doing? (Because I may be a collector of G4s with bad power supplies)

i just sent my G4 back to the reselller
(it had a 90 day warranty)

they initially tried to blame the fact the power supply
failed due to me using an external usb drive with the machine !?
....but when they realised i wasn't completely stupid
they agreed to repair it for me