> A few (2?) public betas of MUI 4 was released after that over a few years

It were at least 19 public betas.

http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4756&forum=9&start=7
http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4383&forum=9
http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4660&forum=9
http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4992&forum=9
http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=5512&forum=9
http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=5554&forum=9
http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=6314&forum=9
http://www.pegasosforum.de/viewtopic.php?t=3241