I am no expert for MUI history so someone else can correct me if I am wrong. How everyone knows was and is MUI the software from Stefan Stuntz who later became part of the MorphOS team and MUI became integral part of MorphOS (the desktop is based on it). Stefan Stuntz developed 3.9 and the first version of 4, later that development was taken over by others. As far as I know is Stefan Stuntz not actively developing anymore (most of the time travelling around) but of course I do not know that just guessing. So version 4 was done by the same developer. The devs from AmigaOS camp got access to 3.9 code (lately it was indicated that they even had access to early 4 code) and were allowed to take a snapshot of that. So code of todays MorphOS MUI4 has nothing (or at least not much) to do with "MUI4" on AmigaOS because the devs there only got the snapshot and had later no access anymore and try to add MorphOS MUI4 features.
MorphOS has been based on MUI5 for quite a while now.
