Hi,
That's wonderful about how early your son learned to read. I bet he will help his little sister to read too.
No matter how brilliant and lovable our children are they are constantly changing so we need to change and learn with them, every age and child presents their own challenges. I don't think there is any negative to early learning-
I have a 4 year old son that sounds very similar to yours. I'm sure we could share some funny stories.
1) My son also wants to do grown up things, he's postive he can drive the car ( I think I have him convinced to wait till he's 16, but I'm not leaving the car keys lying around

) sometimes he even tells me he is going to work, tells me about his office all kinds of details, starts heading out the door to go to work . . . I just listen, acknowledge him and distract him from actually going out the door. Sometimes I play along 'OK see you later have a great day!'
I've had to talk with him about what he thinks of himself vs how other people see his body.
As far as the going beserk when someone calls him little, I have had similar experiences. Kids have their own opinions and points of view just like adults -- they just haven't figured out the social way of handling it when someone's ideas don't align with theirs.The way I have handled it was to talk to him about it, then practice an appropriate way for him to handle it. . . Make sure he's well fed and rested and ready to listen, then bring up the subject --- I know you don't like being called little, or remember the other day when the lady at the store said . . . ? Let him say whatever it is that he thinks about it no matter what it is. Then tell him that you are going to show him a different way to handle the situation (you can even ask him for his own solutions) Then practice a less emotional response. It can be as simple as you pretending to be the lady and saying whatever sets him off then him saying I know you think I'm little but I think I'm big or I'm not little I'm 4 years old! I really make it into a game so he has fun practicing it. If that is too hard for him then start with him pretending to be the lady and you showing him what he could say. Then when the situation comes up and he doesn't go beserk I really praise him for how he handled it.
2) Because we lived in a rural area where there were not many kids his own age he was not exposed to many children until about 6 months ago when we moved to the city. He was more comfortable around adults and would gravitate toward them or play by himself rather than try to interact with other kids his age -- he loved playing with older kids.
When we moved to the city I put him in a preschool so he could get experience with kids his own age - at first he tended to play by himself or just observe ther other kids from a distance. He had ALOT of questions about why the other children would cry or act certain ways when he came home - he just didn't have the experience of what other kids his age acted like - nobody had ever just grabbed a toy from him or cried because they missed their mommy --- now he is just as happy to be with children or adults.
3) What exactly is the situation with his motor skills that you feel needs to be handled?
Perhaps if you are more specific someone can post some other suggestions for you. Below is a link on what is age appropriate.
http://www.greatschools.org/special-education/health/724-developmental-milestones-ages-3-through-5.gs