I don't understand this at all. Did he have four subs in the past and lie to them? Or did they know about each other? If he lied, then, one, props to him for disclosing it to you now. People do grow and mature, and change. Please be sure that you are not speaking to him from a place of punishing him for his past bad decisions, but that you are clear about how very important honesty is to you, and that you really appreciate that he was honest about this and want him to keep on being honest with you.

If the subs did know about each other, then what's the big deal? If monogamy is important to you, make that clear, and then let him tell you whether he can give you what you need. If he can't, better to know now than later.