R1, looked at solely from the standpoint of breathing room for the rear ports, 3-4" should be enough. I've seen a rule suggested of twice the diameter of the port, but this probably implies precision in something which isn't precise. A general rule as to positioning is to have the woofer cones at dissimilar distances from the three nearest room boundaries(i.e. floor and two nearest walls).

-----------------------------------

Enjoy the music, not the equipment.