*Adding this: keep in mind, that gender wise, girls mature faster... in all ways, and are usually more emotionally mature. Boys, develop "later." Thus, don't "expect" him to be like his sister... they are 2 different beings. Boys, develop a LOT differently. I know, I have a boy and a girl. You CANNOT compare them.
The book "Your 4 Year Old" is great. Its a series of books on each age-set, and about what each age is like, so the parent can understand them. Although written years ago, it is still very pertinent and timeless.
He is obviously frustrated... he is having growing pains, which is not easy for him either.
I would also teach him the names for each feeling, ie: happy, sad, mad, frustrated, etc. so that he can communicate that to you.... and teach him "coping skills" to manage his frustrations... with my daughter for example, I even tell her that grown-ups get "frustrated" too, and we are not perfect... but "let's try and work on it TOGETHER...." and then I sit with her and even "let her" get her pent up frustrations out... because it is good for a child to know that they are 'okay' but that we will help them through it.... instead of just putting a "punishment" on them.
Boys especially, NEED to learn how to express their feelings, and that it's okay to have feelings and that someone is there for them.... regardless of how they are. For example: I know as an adult, it makes ME feel better that when I am in a bad mood... that my husband is there for me and we can talk about it without him "criticizing" me for it. Kids, in their own way, are the same.
With my kids, when they are like that... the more I am attentive to them, the less they act out. I will actually pick them up, hug them, show some kind of empathy... and then they deflate.... in time.
Also, make sure he is getting enough sleep/naps. When a kid is overtired, they get more "fussy" and moody. And sometimes, just having a quiet time, helps them to regulate their moods...
I know it's not easy... but 4 years old is an age where they are still "baby" but yet, are expected to act like a "big boy" and sometimes they just can't or are not ready to yet.
They are changing a LOT at this age. So they need to learn coping skills and problem solving for their difficulties.
I also use the "whispering" technique with my kids, and it helps... but only if my child KNOWS that I am present with them and empathizing.
And, kids do NOT like being "compared" to their sibling... it creates animosity and the feeling like they are not good enough or perfect enough.... they want to be themselves. *And, if one child is ALWAYS the one being the "example" it can set them up for hang-ups or frustrations too, or feeling like they HAVE TO be "perfect." And this can add stress to a child. It becomes a BURDEN to them, emotionally and mentally.
And, try and simply ASK your son "why" he is feeling like that. I do that with my daughter from when she was younger, so that she would "learn" how SHE is feeling and learn to explain it to me.... without being afraid of saying so. And now at her current age, she is a VERY articulate girl who is very aware of gauging herself and others... it has given her "wisdom" that is beyond her age.
Good luck, you will get many suggestions here,
Susan