In response to Indigo's comments...

"Would you plan to tell the leader about your son's observed behavior toward younger children?"

Yes of course the leader would know. I was assuming that a social therapy group would consist of children all having some sort of social delays...or else they would not need that group? And I will add, he doesn't always act that way...if he is at a playground he rarely bothers other kids, he is capable of totally fitting in and socializing normally at times. The triggers for the problem behavior we cannot quite figure out. It is by no means, a matter of fact, the hitting of that young girl in his class that was the first time that had ever happened and he has played and worked along beside her daily for 3 months with no problems...I think you are assuming my son is running around hitting and spitting at every single child he sees. No, that is not the case. Those behaviors have occurred very infrequently...they concern me, yes...honestly he didn't used to act that way. I noticed this behavior starting after we moved. I think this has been traumatic on him. He desperately misses our old home where he had his own backyard where I allowed him to play in the fenced in yard and we live in an apartment where that is not possible. My husband also has had to be gone for weeks on end for the first time ever due to military obligations. There may be "normal" child coping things going on here...we have been very stressed financially. He feeds off of the energy of others very much and I now he can pick up on our emotions. It's tough, I've been searching for a job for months and meanwhile we kept them in a school setting for consistency and this has helped tremendously! The behavior really started to worsen over the summer when we moved...sure, he has always had some problems with defiance etc. but his last school where he was one of 22 children in the classroom all ranging from ages 3-6 he did fit in relatively well. Sure, he got in trouble...but it wasn't ever to where they were going to kick him out of school. The other kids didn't let him play with them on the playground sometimes, but he did find a couple of the girls that liked him and played with him. The older girls really loved him and he related well to their kind/gentle ways as opposed to rough boys. He did play well with a girl slightly younger than him also though, so that was good. I never once got a note last year about him hitting or spitting at another child.

Others may disagree, but no, I would not recommend a tablet as the way to resolve a 4-year-old's behavior issues.
I am not saying the tablet will solve the behavior issues, I am just saying it may help his brain stay active so that he can make the more appropriate choices during times that I need to talk with other adults or times that require him to sit for lengths of me, I never thought I would even be thinking of getting one but with the right educational apps I believe it may be a good tool with the right set of rules behind it...very limited use, has to earn it etc...

Yes I have asked his teacher about tips, we talk about it a lot...probably too much in front of him, that's why we need to meet outside of school. But she says that with her boys, she had to have very structured days for them. She does the same for him. A chart is what she recommended, showing exactly what to expect for the day and there are some specific choices outlined in there to give some autonomy/ability to choose activities etc. They always had to clean up after themselves before moving onto the next one. He has a very structured day there, and he thrives on that...It's harder to do at home but I do plan on making a schedule chart and getting some more materials for him to appropriately play with/learn with. When he is engaged, like I have said before he is a pleasure to be around...he asks politely to do things, no aggression etc. It's totally different.

These ideas from your posts plus any impacts of your recent family move, may be good to discuss with a professional.
AGREED. I tried to get our pediatrician before we moved to send us to a professional, and we sat through a couple of sessions with a counselor about child behavior but seriously nothing from that helped mentioned before, if he truly is gifted this may be why no traditional discipline techniques designed for "the normal" child work for us.

baby sister
Because you mentioned his aggressive behavior toward young children, and there is a young child in the home, you may wish to seek professional answers and approaches without delay.

Yes, he can be too rough with his 17 mo sister and I have to watch them together playing 100% of the time because I fear that she will get hurt. He loves her, and they can play nicely together. However, sometimes he just doesn't understand why he can't sit on her, or hit her out of excitement...It's always when he is excited and running around like a whirlwind that he just can't keep his hands to himself. Problem is, usually he isn't hitting her hard just more playfully so she laughs and doesn't cry. He has a hard time knowing limits when other children don't seem bothered by it but the adults around him do...I have a hard time knowing when to step in and when to let them just be kids and play. For him things have to be very black and white...we don't hit PERIOD. I've been tempted to say we just don't touch others but with a sibling that is almost impossible. I am constantly removing him from the situation and sometimes I think he does it for attention also. He will say things like "mommy, look I am hitting her!...come look!" which makes me think he wants attention. It's not usually out of anger, but just rough play or attention getting activity. Still, it's very difficult to ever get anything done when I can't ever turn my back.

Sorry, lots to say...I just think that you think my son is some hardened criminal in the making and I truly don't think that he is. I know you are going to say again that I am defending him because he is my son...but I think that if we make some changes it may not be too late to help him become a wonderful young boy. He can be a delight, super caring and has a huge heart. In all fairness, nobody knows my child like I do and nobody on here can see him in person. A professional will be able to steer us better and give us some answers. I do still appreciate the advice and the insight. It has been helpful for sure.