Sometimes, the youngest one can feel lost in the shuffle.. and his Mommy is busy.
Next, kids this age do do these things... but if it is a problem whereby the school is also taking note and having talks with the Parents, then it could be a case of his behavior is indeed more pronounced than kids normally, at this age.
Discipline... does not work across the board for all kids. Some kids need a different kind of approach.
I found the book "How To Talk So Kids Will Listen, and Listen So Kids Will Talk" to be very enlightening. AND for adults too. Perhaps, this boy needs help with expressing himself and gets frustrated, thus he acts out? "Frustration" is at the core for many kids... and at this very young age.. they cannot know how to handle it. "Coping skills" at this age, is nil. Thus they also act out. It is per their age and development. No matter how "smart" a child is... they are still a child... and they don't "automatically" know how to "cope" with life's challenges.
His other siblings meanwhile, are quite a few years older... thus, he probably can't relate to them or have any playmates... that is tangible enough for him and on his same level. They are not on par with him. And sometimes, when we have older children and then a younger one, we "expect" the younger one to just fall in line... and "know" the routine and follow along. But it ain't so. Youngest kids, also get tired of having to follow along... and tagging along or not having their own time among the hussle and bussle.
There is also a great book called "Your 4 Year Old" which you can get from www.amazon.com ALthough written years ago, it is very pertinent and describes a child per each age juncture and what they are going through.
Sometimes too, kids this young or older, just copies what others do to them... because they are frustrated and don't know any better.
Discipline is not enough for some kids. For my eldest daughter... its not. We teach and discipline (which is NOT the same as "punishment" or denial of something, or an "if-then" logic). For my girl... who is very cerebral and feeling & expressive AND spirited and articulate... we always get the BEST results with her when we INCLUDE her in the "lesson" we are teaching her AND talk about it, explaining very FAIRLY what she is not allowed to do... and what we expect from her... and why. We don't talk "at" her.
Always stating expectations and rationally, and the consequences... then the child knows they are not just being disciplined just because Mommy is "grumpy" for example or "fed up" with normal everyday kid stuff, or that the parent is impatient AMIDST their own "adult" commitments.... ie: for my Daughter, she gets more "fussy/frustrated" when me or Daddy are SO BUSY! She gets so irked... because then she IS lost in the shuffle and we as the Parents are "assuming" that because she is so "smart" and mature... that she can wait it out like an Adult and manage herself (like an adult)... but she is ONLY a child. So as her Mommy... I have to really kick myself in the rear.... and remind myself that she is only a child, and being "busy" is not fair to a child... to a certain extent that it can be helped.
My daughter on a couple of occasions even told me "MOMMY I am only a KID... don't expect me to act like a grown-up!!!!" And yes, that was my bad. SHE was RIGHT on. I needed that kick in the brain from her. But I did not deem her to be a bad naughty girl... but enlightening. If we REALLY look at our kids and observe them, even in the midst of a tantrum... we will KNOW what is bugging them and why... and LEARN from THEM what they need or are missing and what we are not doing for them.
The moral (at least for me): that Parents... can definitely be the "cause" of "WHY" a kid acts out like that. It is not just them acting in a vacuum. It is in an INTERaction... of circumstances and dynamics.
Also, if a kid is always talked about as being a pest/troublemaker etc., then they will be one. Parents tend to talk about their kid in these terms when the kid is right there next to them LISTENING to their conversation... as though the kid is invisible and does not understand "who" they are talking about in this manner. Kids overhear things... it can hurt their feelings or leave them feeling misunderstood... just a thought.
Once I was talking about my daughter (about something just trivial but irritating) and she tells me "MOMMY, I'm RIGHT here.. HELLO! I can hear you! What ab out my opinion too?" Um yah, how rude of me and not nice and disrespectful. But parents do that all the time.
Maybe your friend's son is acting out in school because of stuff IN school... or, it can just be a "displaced" frustration from home or in his life in general. Many kids, act out in school... but the source of the frustration is not from school. Or it could be.
Has anyone just tried and Talked WITH him about it? Seeing what he says or how he feels? And, does he really have time with Mommy? And, for any child, and a boy... having a Daddy time is mega important too. Maybe he needs that too.
All the best, sorry for rambling, just what works for my kids,
Susan