Don't know where you'll be rolling. Suggest 2 fixed and 2 pivoting casters as Burdar has done. If four pivoting casters are used, moving on a slant with one person trying to handle the move can be problematic as casters will tend to pivot down hill.
could use two locking casters on one end and two swivel on the other end. having only two swivel casters will really limit the mobility in small areas.