Fisrtly: One need to have working OS to even make a game to it.. Anything fancifull can be announced years before it'll happen but worrying about such blind shots is a bit too much.

Secondly: Commenting compatibility of partial system (actually: systems as some MOS parts are not final either) is quite impossible. But based on current info it's quite obvious there will be a lot differences on those adwanced features of these OS:es.

Thirdly: Only Software maker that Gains on making OS4 only release is Hyperion (with Hit game they could hope more sales to their own os). All others are more likely to support both systems (or Mos alone cause it has biggert user base and more stabile OS) untill platforms have decent number of sales. Making a game to 100-200 (?) delivered Amiga.ones is quite futile attempt commercially.