These are the system requirements published on the Download page:
· Power Mac G5 (1GHz) or Intel Mac with 256MB of RAM
· 20MB hard disk space
· Mac-compatible Webcam and headset
· Broadband connection
· Tiger, Leopard (OSX 10.4 and up)
So officially the answer is no. From what I understand, a G4 isn't strong enough for video calls but maybe if it has the OSX 10.4 operating system installed it might work for video calls but I don't think a G4 400 MHz would work (maybe a more powerful G4). Other ooVoo features will work on a G4.
Thanks for your interest,
Marvin