You're going to have to change the password for AppleTalk on the G4. Since you haven't mentioend the operating systems you're dealing with on both ends, I'm going to give you the following:
Mac OS 9.x
Click on the Apple menu at the top left and go to Control Panels-->File Sharing. Here, you will change the owner name and the password.
Mac OS X
Open up System Preferences and click on the Sharing pane. Once there, enable Personal File Sharing. Remember that when accessing the G4 from the G3, you'll have to use the username and password that resides locally on the G4 to gain access to that user's home directory.