So what do you base this 'quantifiable fact' on? Is it just that MorphOS runs some software that OS4 doesn't, or do you have some better evidence than this?
Design decisions prevent running certain software on OS4. SnoopDos, MCP, MCX and any other software patching system vectors are out of question. Most 68k drivers are still compatible with MorphOS (although they are no use in Pegasos or Mac). Even 68k Workbench can be used in MorphOS.
Warp3D compatibility is known to be better in MorphOS. Ixemul compatibility is better in MorphOS (OS4 port lacks features like automatic stack enlargement).
WarpUp and PowerUp compatibility is obviously superior in MorphOS but it is not 68k software although WarpUp is part of AmigaOS 3.5/3.9.
68k MUI classes can be mixed with the system. AFAIK in OS4 it is not working.
Other minor details I remember is that in MorphOS MEMF_CHIP and MEMF_FAST are availab le so old software allocating specific memory type is not failing. Certain 68k->PPC and PPC->68k cross calls are supported better in MorphOS.