My sons are just short of 4 years apart. I told my then DS3 that he would be a big helper and told him some of the things he could do to help, and the types of things he could expect - crying, diapers, that he wouldn't want or be able to play for a while and some of the good things like how he would be looked up to and how he could help out around the baby and teaching him things as he got older. He was worried and has never dealt with change well, so we were concerned about how he would handle it. We kept him involved and let him help set things up and pick things out for the new baby as well.
I think one of the best things we did when the baby was born was give him a gift of a baby doll to open when he came to the hospital. It was two fold, a) he got a present so he got to feel special too, and b) he had a doll to feed, and diaper and bathe or whatever else he wanted to do that I wouldn't/couldn't let him do with his brother.
Also, my husband (his step-dad) made sure to give us mommy and DS time just the two of us so he felt he still was special as well.
We were lucky, my older son quickly went from jealous and worried about the change/sharing his mommy and daddy, to adoring his baby brother - and 4 years later they are best of friends (I hope they stay that way!!). I found that late 3 and 4 was a great age to be a little helper he could get me things and follow my directions well.
Best of luck and congratulations!
