I had the same problem, and basically what you need to do is first, obviously, make sure your internet connection is working via your ethernet cable. In other words, is your dsl service provider still giving you internet? What you also need to do in the Airport Admin Utility is to find out whether you are connected with DHCP or PPPoE. You might be required to put in your username and password for your internet service, so if you don't know that, you have to check with your service provider. Once you put all this in, you have to click the update button so the information gets transferred to your airport station. Then it should be up and running.
Obviously, an ethernet cable should be connecting your modem to your airport base station. If you have a router, then you can plug that in via your airport's extra ethernet outlet (assuming you have the version that has one). As for making your two G4' communicate, on either computer press the Apple Key + K and the connection screen should appear. What you need to do is type the name of your second G4 EXACTLY as it appears in your account preferences folder. In other words, whatever name it is you gave to your computer when you first set up your OS. Then you type in your password, and one of your G4's should appear on your second G4's desktop. I know this sound all confusing and stuff, and that's probably why it's best for you to call Apple and they will walk you through it. I called them and I spent about an hour on the phone until they helped me set it up properly.