Same here today with SU4. I’ve got all WUs and CUs installed locally and the sim suddenly wanted to download around 150–200 GB again. During the process it only seemed to actually install a small part of that, and afterward everything still shows as needing an update – so it just loops.
This never happened to me with previous SU updates, they didn’t require a full re-download like this.
Does anyone know if Asobo is aware of this issue and working on a fix? I couldn’t find anything official about it.