with G4's and networking in the past - usually on clients' networks where lots of machines are using Cisco hubs. In those situations I've become used to the phrase 'Ethernet deafness' which seems to aptly describe the G4's behaviour - the network can see the Mac, but the Mac can't see the network. In the corporate situation I usually disable Spanning Tree Protocol on the Cisco hub, and it seems to work like magic.
You're not using a Cisco hub however, and I'm not sure of an answer in that case. There were some common networking issues with Macs a few years ago, but to be honest I don't deal with many Macs in the course of my work, so on this occasion I'm not the person for you. I'll call back to your thread later on this weekend, and if you're no nearer a solution I'll do some research.