Most old G4s PowerPCs have Problems with the Elkos of their Powersupply.
If you're a bit into Soldering, you can quite easily exchange these and get it working again.
Sadly, it didn't work out for me, maybe something different wrong...

BUT - for the sake of helping you out, I made a Picture with the Specs of the Elkos that normally "fail".
Red are the ones which actually were really dead as I measured the old ones (i.e. the 25V 100microF had only 68microF left),
Green were the ones that "still worked" / were in quite good condition.

But thats only from one Powersupply, on yours it might look different.

But anyway, good luck!