Q: my imac g5 turns off after 4 seconds how do i fix

Impossible to answer with information provided.

Does anything come up on the screen before it shuts down?

Does the computer have a newer than 4 year old clock battery (PRAM or CMOS battery is the same thing)?

Have you checked for bad capacitors?  There is plenty of documentation on the bad capacitors that were given this model.

Have you checked the power cable is flush plugged in the back of the machine, and there is no housing of the power cable that exposing the interior of the socket?