Well, think of it this way, bullets from a shotgun hit different parts of the body, each shot will do varying amounts of damage depending on where it hit, somewhat simulates this. 
So lets see.. dividing 5 by 4.. 1 damage per a bullet! |
Then just increase the overall damage of the shotgun if per-bullet damage is too little, which'll mean that shotguns would be the most powerful at close range, which would be in keeping with most other shooter games.