'Is possible to use micophone in G4/733?' - directly, no. Via USB, yes.
To use a standard PC style microphone - you will need to purchase a device such as the Griffen 'iMic'.
Another (possible) solution is a USB microphone, such as the 'Logitech USB Desktop Micophone'.
I purchased such a microphone and tried it on a PowerMac G4 and Mac Mini. It was returned, for a refund, due to its less than satisfactory performance. Your results may vary.