Need help with projectile splash damage

I think this one should be fairly simple since I already have an idea of the command to use. But it doesnt really work so Im asking all of you. I simply want it to cause 4 basic damage to entities in a small radius around where the projectile collides with the entity. To break it down, I want this ranged item to do damage to the entities around the entity that was hit. If someone could help with this that would be greatly appreciated!