Mount them about the same height across the front, on the wall or the ceiling, upside down. Put the center one above the TV in the center. Try to place the side ones at least 3-4' from the side walls.
The rear 2 don't need to be the same height as the front 2 - but each should ideally be the same height as the other rear. Again, wall or ceiling, up 7 or so feet, upside down is often a good setup.
A little hard to be precise given that you haven't described the room.