I am in total agreement with Lisa M. I think some of the other responses are a bit harsh. After all, your oldest is only 4, and he's been the baby all that time.
You should be thankful beyond belief that he's not hating the baby who is invading his space!
So here's my suggestions. I have a 6 yr. old and 10 month old, both boys. When we found out I was pregnant, I conveyed my concerns to my Mom about the age difference. After all, Peanut had been the one and only for 5 yrs.!! So for Christmas that year, she bought him his own special boy baby doll, his name is Jeffrey (so fitting, since he's a Jeff Gordon fan, lol). I bought Jeffrey some bibs, bottles, and my Mom bought Jeffrey some baby clothes (he's the size of a real baby and I've actually passed on some of our 10 month old's clothes to the doll), and blankets.
We began teaching Peanut how to take care of Jeffrey. We even gave him some diapers to use on the doll. When I finally had our little guy, in the beginning, I would have Peanut get his doll whenever Ben needed fed, or changed. And he would mimick the feedings (with a bottle), or changings (with new clothes and a diaper). He would sit and rock Jeffrey, while I rocked Ben.
Peanut has been in love with his baby brother since day 1, as well. In fact, he loves to tell me that HE loves Ben more than I do, which I just laugh at, lol. So I incorporate Peanut's help with Ben. He helps get diapers, wipes, blankets, keeps Ben occupied while I change him or am getting his food together.
I think you might want to try the doll approach and even the big brother helping MOMMY approach. And explain to him, NICELY, that you appreciate his help, and that you understand he loves the new baby, but that baby's are sometimes scared when people get too close. You don't need to be harsh or mean about it. Just remember, this little being has taken the only child's place. So you don't want to hurt your oldest child's feelings. After all, he just wants to help and be part of the baby, which is very good.