Are we not answering the question you are trying to ask?
The feat allows the character, as a full round action (note that he cannot move AND use Rapid Shot. nor can he take any other move-equivalent action) to make one extra attack (at full BAB) at the cost of a -2 penalty to hit on all arrows fired. At L4, with no iterative attacks, that's two arrows instead of one.
Each arrow gets its regular damage, so 1d8 + 5.
Adding the bonus only once to two (or more) arrows combined wouldn't seem fair to me - multiple melee attacks get the full magic bonus and at least half the STR bonus. It would also be confusing - what happens if only one arrow hits, or if he fires arrows at two different opponents?
Is there some third alternative we are overlooking, Asmo?