This is a basic code, but it does what you want.

Code (Skript):

1. on right click with diamond hoe:
2.     if name of player's held item is "&6Shotgun":
3.         set {shotgun.bullets.%player%} to 4
4.         shoot snowball from the player at speed 2
5.         push last shot snowball upwards with speed 0.06
6.         shoot snowball from the player at speed 2
7.         push last shot snowball downwards with speed 0.09
8.         shoot snowball from the player at speed 2
9.         push last shot snowball left with speed 0.06
10.         shoot snowball from the player at speed 2
11.         push last shot snowball right with speed 0.06
12.        
13. on damage:
14.     if event-projectile is snowball:
15.         {shotgun.bullets.%shooter%} is 4:
16.             damage victim by 4