Four-year-olds and two-year-olds are not going to be "good" all the time. I am sure that your husband has unrealistic expectations regarding their age-appropriate behavior and their ability to behave like he wants them to. As opposed to saying that he is wrong and you are right regarding how you discipline your children, suggest to him that you both need to find better ways to parent your kids, especially in the arena of discipline. Your husband is screaming at your kids because he wasn't taught a better way of dealing with them. You've heard the phrase, "When you know better, you do better." Rather than tell him he is the problem and he needs to get help with his temper, suggest that you want to see a family therapist with him and maybe take some parenting classes, too, in an attempt to come together on how you parent the kids. Offer him praise for what he does well with the kids and admit to wanting and needing some help yourself. Tell him that you want to feel like you are a good team, that you support each other. Once you begin therapy and parenting classes, he'll have the opportunity to learn different techniques of disciplining your kids.
Telling him that he has to change or you'll leave him will not bring about the changes that your family needs. If he refuses every invitation of yours to go see someone together and continues to emotionally abuse the kids, then you'll have to consider whether your kids will indeed be better off without his daily presence.