I believe my 4 year old has ADHD. We took him to a Psychiatrist who gave us that evaluation in one visit. I definitely know there is something more there than just 4 year old behavior. He had a hitting problem starting at about 1 1/2 that only ended recently. But he seems so angry and frustrated. Also had him in sensory integration . He has a problem reading social cues, hyperactive and defiant. You have to tell him the same thing 10 times . He has meltdowns about what seem to be insignificant things. He al;so has situations where he needs you to do what he says "exactly " how he says it. from telling a story to walking with your arms a certain way. It is so much work. I also have a 1 year old. I don't even feel good about leaving a sitter with both of them. Because all she will end up doing probably is to keep telling him to leave her alone. Not to mention my neighbors have ousted me because of past behaviors they deem unpredictable. If someone plays aggressively "like 4year old boys do" He takes it to the next level. He is very sensitive if someone else gets hurt or if someone doesn't want to play with him. He feels so deeply.
I am in desperate need of support for this age group.
Are you sure you're not describing MY son?!?!? He sounds exactly like mine! My son is ADHHHHHHHHHHHHHD and so severe that he started medication just before his 4th birthday. I know this seems very young (and it is!) but we just couldn't wait! His self esteem was getting so bad at such a young age that the doctors didn't even argue with me! We were so desperate
!
My son also has sensory integration problems-auditory sensitivity, vestibular problems, and sensory seeking with touch. Also, he doesn't feel pain like other kids so he doesn't really know when he's being too rough. This used to be a real problem around other kids.
The medication has literally been a lifesaver for my son! We still haven't gotten it "perfect" but he can function now! We can actually take him out of the house and go places. We couldn't do that for a long time. We tried preschool when he wasn't medicated and he couldn't do it so we had to take him out. This year he's medicated and is doing MUCH better in school.
What did his psychiatrist recommend for him? Hang in there. It WILL get better as long as you stick with it and find a doctor that you like. Good luck! 
Did you have a evaluation? We went to a pediatric neurologist at children's hospital just shy of 5. DS was diagnosed with severe adhd BUT we didn't begin meds until he was 7. Biggest mistake of his and my life. He wouldn't have struggled socially if he had the meds tohelp him. They really have.
I would contact children's hospital and schedule an evaluation. NOt all doctors believe they can diagnose 4 yrs olds, if you can believe that one. Nor do some doctors not believe children are bipolar!!
you described my son to a T! He has been on medication since he was 3