Is there some incompatibility between 256 QAM and 4T4R (Rank 3 and Rank 4)?
Yes, as I can recall in old releases like till L16 it was there.
256 QAM and 4x4 MIMO was not supported togather.
Not sure from which release both are compatible.
Ericsson Experts can comment.
Limitation is planned to be removed in L17.Q4