Stream movie on Apple TV 3 from iTunes on iMac G4
http://support.apple.com/kb/HT4437 says iTunes 10.2 or later is required.

If your iMac G4 can run OS X 10.5 (Leopard) you should be able to run iTunes 10.3.1 (http://support.apple.com/kb/DL1103) which qualifies.

If it can only run OS X 10.4 it appears that the latest version is iTunes 9.2.1 (https://discussions.apple.com/thread/4108495?start=0&amp;tstart=0) which does not qualify.

Note that I have not verified the above.  Also, based on personal experience, you want as good a network connection between the source and the AppleTV as possible.  Either cabled or 11n wireless.

Good luck.