There are some amazing women here aren't there? So much great advice. But advice is only good if you take it and use it.
Your son is 4. He will act like he is 4. He will do things because he is 4. If he is "out of control" it it because he is 4. (By the way, who's "control" do you want him to be "in"?)
Do you spend time with just him?
Do you honour him the way you honour the new baby? You know you spend one on one time while breatfeeding, when does he get equal one on one time?
Do you do special things just for and with him?
Does daddy and him have quality time together?
Does he get treated like he's 4 or do you expect him to be more grown up then he really is?
Why does "most" of the attention got to the new baby? There are two parents in the house and 2 children.
Does he have something special that is only for him, that only he can do, have or touch when you are breastfeeding?
Does he know, really know deep deep down that you love him the same even though you spend "most of the attention" on "the other child"?
I hope I am not coming across as being mean or anything, i would just to get you thinking about what could be different.
I have a 12 year old and a 9 year old. Right from the beginning, they were treated to one special "thing" each week. So for example, when I was breastfeeding my youngest, the oldest had her "grown up dishes" and she got a snack. She got "real dishes", "real cutlery" etc and was able to "get her own snack". Then, as soon as the youngest was asleep, the oldest got 30 mins of "her time". There was never any jealousy, there was never any sibling rivalry. There was no need for dicipline.
If you know that he is jealous, change it. If you know he feels he gets no attention, change it. You are the mom. You make the decisions. You can to choose what kind of house you live in and what kind of children you raise.
You are a great mom. You asked for help. That shows you truly care.
B.
They still have one special day with mom and activity (seperately each week).