Power and control. Power and control. Having worked in a daycare for several years, I know that sometimes a child feels like the only way he has control is to force it on another. For whatever reason he is doing this, it works for him.
Whenever someone hurts another, the general rule we tried to follow, is to go to the "victim." I'd make sure that FIRST, your 3 year old is picked up right away, and encouraged. For that moment, your older son needs to see that "It isn't working" for him. That he actually caused you to direct attention somewhere other than him.
Later, in a private manner, state, restate, whatever, that hurting others with our words, hurts their heart, and that you will not tolerate that behavior. What is his "currency?" What are the things that your 4yr old loves to do or have? I have a friend who allows her son to watch a half hour Thomas video if he gets smiley faces for obedience, or good behavior, etc. If he choses not to obey, or behave well, then the smilies go away and frowns take their place. Then HE IS in control of getting the desired movie at the end of the day. It works well for him. When you hear him use his words in a very positive way, then make sure you verbally say how proud you are of him, and that he is going to be a great daddy some day, or something like that. Kids need to know they are becoming something wonderful when they share good behavior, so say something like, "My, you are a very ______ young man." (Kind, thoughtful, helpful, nice, encouraging,etc.) Don't overdo it, just tell him plainly, and use the appropriate word to the situiation. If he sees himself the way you see him, he will definately try to keep that image. If you really want it to sink in, retell your DH in front of your son, what he said and how thoughtful it was.
I hope something works for you, and the others will have something else to add for you. Best of luck.
J.