S.,
First, read the book "Raising Your Spirited Child"...it has a lot of tips for children who are slow to change. Basically they need to know about things early and how it will be fun for them or to their advantage. You need to get his buy-in.
What I took my oldest to a Sibling class, which talked about how helpful he could be and the things that he could and could not do with/for the baby. See if there is one offered in your area.
Look at and read books about the baby's growth and development (our baby is this big and has tiny fingers, etc.). Also read books about being a big brother...there are lots of great ones; check out Amazon. One of our favorites was My Big Brother by Valorie Fisher, shows the importance of the big brother from the baby's viewpoint. It's SO important that he feels included and gets excited about the baby because so far it has always been all about him.
Tell him all the ways that he can help you during your preganancy, especially when you get bigger...perhaps picking things up off floor for you or helping put on socks or tying your shoes; my 4yo son did this for me. If he reads, he can sit on your lap and read to the baby. Create a bond now.
Also talk about how he can help with the baby...patting the baby when it crys, retreiving diapers, wipes, pacifiers, toys, choosing clothes, holding baby at times and helping "watch" the baby. My son was a little jealous at 4 with a new baby, but the more I got him involved, the more helpful and loving he became. You need to let him in on the reality of the amount of your time the baby will need, but let him know how he can signal you that he needs mommy time so jealousy does not grow. Include him in everything!
Regarding the move, tell him as soon as possible and let him know how he will be able to have his own room still instead of sharing with the baby. If you can afford it, allow him to "redecorate" his room or pick paint, etc. Also let him help pick out the theme or colors of the baby's room. Maybe he could even help paint or something.
I have to say that the transition was a little rocky at first, but I could not be happier with the 4 years between my two sons. At 4 they are beginning to become independent any way...make a big deal out of all the things your older one can do that the baby cannot. With your older boy helping he will also gain a lot of responsibility, so make sure to praise and reward him.
My sons are good friends, my oldest rides his bike to school with the younger one (now 13 and 9), watches over him and tries to teach him responsibility and keep him out of trouble. It is so sweet! Enjoy!!