A character cannot normally attack targets in his back facing, and attacking targets in his side facings are at -2.
Attacks TO a target's back facing (or from an unnoticed attacker, including one above, below, or at great distance) are at +4 to hit, with attacks to the target's side/flank being at +2.
Facing modifiers are cumulative, a target with its back to the attacker's flank is attacked at +2.

Play Rules