I think higher order of MIMO can be used. I remember a paper where an author demonstrated the performance for a 128x128 or was it less (poor memory), but it was certainly greater than 4
I think the performance improvement is not that greater when the number of antennae is increased to greater than 4 i.e. it might be the optimal number
Because increasing the antenna number have a few disadvantages. First of all the time for transmitting one block increase and system gets more complex.