Hyperion have released a SDK for OS4.0, but I'm not sure whether it can be used on OS4 only or whether it has the cross-compiler stuff too.
Although I'm not a fan of it myself, CubicIDE (formally GoldEd) seems to be the easiest way of getting started and IIRC it contains all you need for MOS OS4 and 68k.
AROS also had some interesting tools if I'm not mistaken.
