My G4 is having the same issue. I noticed it when I was trying to transfer something over FTP and it wouldn't connect to my MacBook like it normally does. I checked in the TCP/IP control panel and sure enough, it had a self-assigned IP address. I use an Airport card and it has worked flawlessly up until now, so I don't know what is up with it. If there is any information I can provide about my machine or what is running on it to help with finding a solution, I'd be happy to provide it. Nothing significant has changed in my setup between the time when it did work and now.
On a side note, I have Mac OS 10.4 also installed on here, but the disk is partitioned and I have set it up so that OS X cannot see OS 9. In OS X, networking is operating as it should.