The fastest clocked Apple-shipped G3s were the 12" and 14" iBook G3 in 2003 with a 900MHz 750fx G3.
I expect the G4 Mac mini would be in the running for quietest G4. With the hard drive spun down or an SSD installed, my 1.42GHz model is extremely quiet at idle and light loads. I've found the late model aluminium PowerBooks tend to heat up more quickly and spin up their fans sooner. Once going, they both sound like hairdryers though. The iMac G4 makes a bit more noise at idle, but doesn't go hairdryer/leaf blower under load. I like my 800MHz Superdrive model with SSD installed.
I expect the G4 Mac mini would be in the running for quietest G4. With the hard drive spun down or an SSD installed, my 1.42GHz model is extremely quiet at idle and light loads. I've found the late model aluminium PowerBooks tend to heat up more quickly and spin up their fans sooner. Once going, they both sound like hairdryers though. The iMac G4 makes a bit more noise at idle, but doesn't go hairdryer/leaf blower under load. I like my 800MHz Superdrive model with SSD installed.