I like the idea of AROS on pi because its doable, and satisfies bigger ambitions. It's API and binary compatible with OS 3x - and has the opportunity to get loads and loads of users if marketed correctly. It can be skinned up to look like 4... the icons are open source after all (but I'd suggested its skinned up to look modern - a lot of Amiga stuff still looks like its from 1994)
There seems to be no interest from the owners of OS4 to port it to the Pi... its stuck on hardware that is expensive and is harder to obtain.
Surely growing the userbase is a primary concern? reintroduce a new generation to Amiga. Port a good browser over - and build from there?