Howard Nurse, W6HN
The existing Audio and Keyer boards will work just fine with a Raspberry Pi 4B. The problem is the connector locations on the new 4B board which require a modified case.
I've tested the 2GB up to 8GB Raspberry Pi 4B boards and haven't noticed much, if any, difference. You can use htop from Terminal on the Raspberry Pi desktop to see how much memory is being used. It is rare to see the 1GB RAM on the 3B+ exceeded causing the swap space to be used.
Note the 1GB 4B is no longer offered. The 4B 2GB version runs about $35 from all the usual sources.
Correct, the Raspberry Pi 4B can't use the older O/S (Stretch), it requires Buster. Buster will run on the 4B and 3B+. RigPi 2 uses Buster.