Finding out about Cheating can be an extremely traumatic event with potentially severe consequences. Maybe not for a 16 year old but I won't be so sure. If it's his first relationship then his trust will be completely broken and It may become difficult for him trust people in future relationships or a much more severe reaction would be that he starts developing misogynistic tendencies because his poor experience with your daughter.

I don't know how mature the kid is, but 16 is still quite an impressionable age. I would give the kid a break and just let him know if your daughter refuses to break up with him. If you don't do anything, you are also teaching your daughter that it's okay to be unfaithful and there are practically no real life consequences when the reality is quite different.

Mistakes are mistakes only of we learn from them and move on. Making the same mistakes repeatedly without changing, is called a habit. Your daughter is developing a habit of cheating. Once or twice can be chalked down to a mistake. But 3-4 guys at once, That's tough.