I recently put a new battery in my 911 C4 and had to dig up a code since I am not the original owner; I found this thread and thought I'd pass along what I wound up doing:
I got the model and serial number by using the "hold down the TP button for 10 seconds and then use the right hand knob to display".
I found
Porsche Resetting Radio Codes which has a Java-based code generator. It requests just the serial number in one box -- press the "Decode" button after entry. It
provided multiple choices of code, "first choice, second choice , etc" and the first choice 4-digit code worked for me.
Thanks to all who posted on this topic -- very helpful!