J.G.
When my 4 1/2 year old gets like this, I get down to his level or pull him on my lap and talk to him. I start out by saying, "It's really hard to have a little brother and have to share all the attention, isn't it?" And he gets that sad look in his eyes and nods and says, "Mmmhhhmmm." And then I hug him and tell him how special he is and how much he means to me and some of the fun things that he can do that his brother can't.
He proabably does feel like he's not getting enough attention. And it's really hard because parenting is exhausting, and we often feel like we have nothing left to give!
I've found that if I acknowlege my son's feelings and help him see all the attention he does get and all the great things we get to do together, his behavior changes. He even begins to appreciate his brother again.
Maybe he just needs to feel that you understand what it's like for him and needs to be reminded of how precious he is to you.