Assuming you are referring to infantry units I would support such a change. I have found that long spears in units of four get swept away quickly by devastating chargers in Magna, this change would go some way towards countering that.
Alternatively, and preferably, in order not to have an artificial difference between units keep them in fours and allow infantry long spear units in Magna to keep their +1 combat bonus when only one rank deep. This can be justified by the assumption that individual bases in Magna represent more men than bases in Maximus so are in more ranks.