Slitherine

A unit defending an building has rear support, how does the dice addition affect combat,

Say two hits are assaulting they get 4 dice each the defender 4 dice against each.
Does the defender get an additional dice against each attacker (5 dice against each) or 4 dice against one and 5 against another?
Does the attacker lose 1 dice from each attack (3 dice each) or 1 dice off the overall dice (one of 4 dice and one of 3 dice)?

This is covered right at the end of the FAQ on buildings.

All the attackers lose one dice, and the defender gains one dice against one opponent.

Also, remember that the unit giving rear support doesn't actually need to be 'behind' the building, as determined by the direction of attack.
It only needs to have some part of the building to its front, and be within one move and 6MU.