Yes, he is transferring his "frustrations/stress" onto another child, Your Son has found someone to take his frustrations out on. No, it's not on purpose... a child so young does not "know" this. They need help in navigating through their emotions.
Having a new sibling is definitely a cause. Your baby is only 4 months old... it takes time for an eldest child to adjust to things.. to sort through it, his feelings, his ideas about it.. his confusions, his "new" role in the scheme of things.
No, don't approach him as though he is a "bad boy." RATHER... you need to sit down with him, and re-focus on him. He is 4 years old and by this age....he can certainly "talk" about things. LET HIM TALK about anything with you, without judgment. I think he is pent-up inside and his insides/emotions are hurting. Thus, frustration and anger. All kids need to "vent" just as adults do....they are tender and just as prone to "stress" or frustrations as adults are. No matter how bright or articulate or social a child is.. they need understanding. He is normal... having a new sibling is a BIG deal to a child.
Yes, as you say, he can "make" peace with your baby, his sister. BUT it takes nurturing and guidance from you/Daddy. I used to make extra sure that I did not loose my girl in the shuffle when I had my 2nd baby... I paid close attention to her moods and behavior.. and even though it is busy with 2 kids...carve out "special" time with just only him. A "Mommy & Son Date" so to speak. They need this. And, with my girl, we always emphasized "teamwork" and that we are ALL special to one another, always.
Kids, (boys especially), need to KNOW that it is okay to talk about how they feel. At anytime, with Mommy or Daddy... and that they will be validated.
Just as an example: I was once at a BBQ party with some other families. A 7 year old girl (which I knew) sat down by me... she then started to totally VENT about her life and how miserable she was. She also told me in exact words "I'm so stressed...my parents never listen to me...." and she went on saying how SHE is always expected to do everything perfectly & if her younger siblings are upset about anything SHE is always blamed since she is the oldest etc. And that she always has to help with her siblings which she does not like because she wants to play & see her friends and she has homework to do etc. -I told her she should speak with her parents about it... and she said "No...they never listen to me or believe me. They never like anything I say...I just have to do what they say" Now, this child was VERY stressed-out and unhappy. Mind you, her Parents are highly educated Professionals in the community. BUT...this was their eldest child talking... to me. She had lost all confidence in her own Parents and had lost all "trust" in her own Parents that they would help her. It is very sad.
Any child needs to be heard and listened to... and observed to see how they are handling daily life or changes in their life.
Sometimes, "bright" children are often assumed to be fine...because they are bright and articulate...BUT "emotional" competence of a child is a whole other issue. My daughter too is very bright and articulate and mature for her age...but, she is still just a child.
I'm sure your little guy will be fine... but spend time with him...show him how his little sister LOVES him too... and how they are a TEAM. In time, perhaps your son will adjust to having a sibling. But you need to nurture the relationship between them....positively, and through examples. This is what we have done with my eldest daughter when I was pregnant and had my son who is now almost 2 years old. Always include your son in anything... he needs to feel that he still fits in maybe...with my eldest child... I tell my daughter, in our private moments together that "You will always be my FIRST baby.. .and Mommy loves you..." My girl seems to get very happy when I tell her that, it makes her feel special.
All the best, your son is still adjusting to his sibling and needs acknowledgement.
It'll be okay... but address it now.
Good luck.
~SUsan
It will be okay...but