@hooligan
WinUAE as a dev environment.
Actually, providing a WinUAE base configuration as a condition for the bounty makes sense for classic development, as it's accessible to almost everyone. It should be possible to put together a pre-built image and build system, a la AmiKit, that prompts for the Workbench 3.1 disk images during installation.
AROS is open but constantly changing, so it would be difficult to define a baseline.
I'm exagerating, but I think the Amiga OS 4 community is something like this: 0.01% - developers with OS4; 0.01% - developers without OS4; 99.98% - prospective users (some of which probably don't even use their PowerPC hardware). Would be developers do not have access to hardware.
MorphOS, at least, is viable in the sense that developers have ready access to hardware.
All that said, Amiga OS 4 should probably be off the table as a target platform until hardware becomes available.