How can I create a transport request for a trading partner (WE20) in SAP 4.7? It looks like SAP has taken off this functionality.
No! That would not make much sense. A transport request copies data from one system to another. The partner profile defines a route from sender to receiver. Hence the target system of your transport request (call it PRO) is ultimately different from the system where you defined the routes (call it DEV), the data would be false, as they would still contain the info that data is sent from DEV to somewhere. What you want in the target system is: send from PRO to somewhere.
In addition: the partner profile WE20 is something like a key lock or the final valve to switch the connection to live. Typically you would not want to copy a transport request that automatically puts live a connection to external data. Data sent from DEV will not really go to the same receivers than the good ones from PRO.