Is the Router using WPA2 perchance? A G4 cannot do above WPA.
Make a New Location, Using network locations in Mac OS X ...
Is that Interface dragged to the top of Network>Show:>Network Port Configurations and checked ON?
The Interface that connects to the Internet, needs to be drug to the top of System Preferences>Network>Show:>Network Port Configurations and checked ON.
Then... Try putting these numbers in Network>TCP/IP>DNS Servers, for the Interface you connect with...