Why do my 4 puppies bully their youngest and smallest sibling?

Well dogs are very dominant animals, so what you can do when they are treating it so badly is take the runt and give it a lot of attention that shows that you care for that dog and if they believe you are the dominant then they will have more respect for the runt. I used to have to do it with my little pomeranian and now i have the best relationship with her haha.