After what you wrote, I repeat that I do not really know. From the comments, I conclude that only few know exactly how it works. Many experiment, but I want to stick to academic knowledge at first and only then experiment (eg with Fresnel / Arago / Stand waves specific).
I previously assumed that 4x4 MU-MIMO is a group (possible in a different horizontal plane) and in a group it works according to the lowest MCS (if someone in the group has only MCS2 then it will be the whole group), therefore, MU-MIMO is used only if if SU-MIMO can no longer be effective (the AP itself calculates which option will be more efficient).
The Cambium forum has WISP, which with horns placed on their side, trying to realize 4x4 and MU-MIMO - https://community.cambiumnetworks.com/t/epmp-3000-ap-and-dual-horns/67660
How is it really there - 4x4 SU-MIMO with THB, 4x4 with horns in different directions, with Cambium Dual Horn (I heard you developed) and MU-MIMO with / without band? Can you tell us now without waiting for a webinar, video, etc.?