Faster Answers

I put in a new PRAM battery and pressed the CUDA or SMU reset
button combination. Made sure the HDD was okay.
I don't have any extra RAM to try swapping - did not try that.
Pretty certain the CPU is okay. Sure wish I could test out that
power on button assembly/power supply, but I can't. The only
other thing it could be would be the main logic board. Any ideas?

Does the green ring light up when you plug in the power supply? Does it turn yellow when you try turn on the Laptop? If the green light does not turn you,
you may have an issue with the charger. If it the ring intermittently turns yellow when you move around the plug, you may have a broken DC-Jack module. If the ring stays solid green regardless of what you do you should open the laptop and short the two contacts for the powerbutton connector on the logicboard (e.g. with a screwdriver). If that doesn't turn it on you ruled out the power button and you're down to the logic board. Do some research on logic board repair for the G4 and you'll find there is a common error with a faulty solder connection on a small DIL chip located half way under the cable between the DC-In module and the corresponding connector on the logic board. Resolder pin 1-3 of that chip. Still no success? You can inquire into reflowing or replacing the logic board.