Thanks for taking the time to compile that explanation of Fuji's IBIS/OIS situation. Pardon the following lengthy backstory here, but it serves a relevant point.
As a hybrid shooter, I came to Fuji via the X-H1 (Fuji's first body with IBIS), from the Panasonic G9, because I much preferred Fuji's colors and dynamic range, on which Panasonic's M43 cameras were weak. What they were strong on, however, is/was IBIS, and the Lumix Dual IS (IBIS/OIS conjunction) I found wonderful, from its Steady Shot locked-on static shot to its ability to pan without jarring and stuttering. With the X-H1, I was, by comparison, disappointed with its IS situation. When using non-OIS lenses in shooting video, the IBIS was nice for hand-holding, and could sometimes handle panning and motion, such as following a car on a street. With OIS lenses, however, I had to forget about panning and tilting video shots. It was dismaying that I couldn't select "either/or/and" for OIS and IBIS. For stills shooting, however, it was great to be able to hand-hold at slow speeds. Since the X-H1 was also the first Fuji body to feather the quiet, near-shockless electro-magnetic shutter, which also encouraged my transition from Panasonic (whose cameras since the GX85 have had a similar shutter mechanism), there was less for the IBIS to have to stabilize than with the "older" Fuji shutter.
I got the X-T4 because of its switch-segregated Still and Movie modes (almost/not-quite implemented on the X-H1), its continued use of the electro-magnetic shutter, the larger-capacity battery, its being the first Fuji to write video files redundantly to both SD cards, and the body style of the X-T2, which I also owned and which I preferred. Adding the SmallRig rubber grip approximated the grip of the X-H1, when I'd want it. I resented having to have the "flippy" screen – because I don't shoot video of myself, I shoot video no differently than I shoot stills, and I've seen too many LCD screens on Canon cameras broken-off at the swivel-joint – but for everything else about the X-T4, I considered it a necessary nuisance.
Along with all this about the X-T4 I expected to come improved IBIS, as Fuji would surely have ironed-out the glaring kink in it. But no, it was about the same; maybe-maybe improved, but still an impediment to non-static video shots. I also wondered why Fuji wouldn't enable the user to de-couple and selected between OIS and IBIS, which would seem possible within firmware. I also began to notice that with telephoto zoom lenses with OIS, the IBIS would struggle against it, resulting in mis-framed shots, both in stills and in video. My solution to this became to shoot still images with a telephoto zoom lens (most recently, the 70-300) on an X-T3, which doubles for the X-T4 in my hands and serves as back-up if the X-T4 goes down.
Another thing about the OIS/IBIS combo is that, when I was shooting with the X-T4 with the 16-80mm zoom, from a moving ferry boat, fixated on an island in the middle distance on the water, the rapidly-moving water in the lower third of the frame caused the IS to jump and jitter, as it was confusing the IS. If there were some selectable panning mode to the IS, as on Panasonic cameras, this might not have occurred.
In video, the "+DIS" IS mode on the X-T4 does help with static shots, with a negligible crop penalty, especially in windy conditions. However, I've found that usually I can shoot hand-held static shots without it added, and stabilize them in DaVinci Resolve, resulting in shots that look as though I'd used a tripod. I just have to factor-in a bit of a crop, but I tend to shoot too wide anyway (with the 16-80mm on the X-T4 80% of the time).
Fuji's OIS itself is great for stills shooting, but the IBIS is far greater for video. I wish that Fuji would consider the issues such as I've described (and I'm hardly the only person who has mentioned them online) and remedy them with some firmware updates (going back to the X-H1, if possible), although at this point, they would probably reserve that development for future bodies as an inducement to buy the new models. Fuji should realize that "video" doesn't mean "vlogging," and that not only do video images move within the frame, but that the frame itself also moves.