I know the OS4 port of SDL is being developed by Hyperion but I don't think it's fully ready for release. If you develop using the 68k headers it should be source code compatible with the OS4 native version.
--edit--
If you really need to know you can email
MagicSN for all the details. (He's the guy at Hyperion that ported XMAME SDL to AOS4.)
