Over a week or so, make a note every time anyone in your home screams or yells at someone. It will be easier for you to help your child if you see he is not the only one who is behaving inappropriately. If you have a quiet family, then count the times anyone raises his voice or talks harshly to someone. This is your family's brand of screaming and yelling.
The activity will help you see the specific child's behavior is inappropriate only by degrees. He yells and screams too loudly, too much, too long, and at the wrong times. He over does it. That is what his problem actually is.
Start your helping process by being sure no one yells and screams at your child when he yells at someone. Each person in your family needs to take responsibility for how he handles the specific child's behavior. At a family meeting, you can set the rules. Say, for example, "Kevin has a problem with screaming and yelling. We all are going to help him learn better ways to say what he has to say. Let's agree to do this. Any time Kevin yells at one of us, he or she will wait patiently until the yelling stops. Say this to him. 'If you've finished yelling, I'd like to hear what you want to say to me. Will you tell me in a more appropriate way?'" Just be sure you are ready to be good campers when your four-year-old reminds you about the family rules by saying to you, "No yelling. I only listen when you talk right."