You need help NOW, before that new baby shows up. Your 4 year old is about to have his world turned upside down,
this is really a huge transition, and it's almost always rocky. He'll need lots of love and reassurance, and you'll need lots and lots and lots and LOTS of patience.
First, I want to recommned two books to help you with "typical" four year old issues (not related to the "new baby" issues). Once the new baby shows up, you'll have NO TIME for reading, so go online now and order these books:
"Positive Discipline for Preschoolers" and
"How to Behave so your Preschooler Will, Too."
There's also a good one called "Siblings Without Rivalry", but it's mostly geared toward moms of older kids. It's a classic now, so you can probably get it used on Amazon for just a few dollars.
There are lots of books for children that are about to become big brothers. We really liked "Little Tiger's Big Surprise" - I hammed it up and make funny voices and faces so it was silly fun. "On Mother's Lap" is also sweet. And "My baby brother is a monster!" lets older kids know that it's OK to be angry. Lots of kids say things like "I wish I didn't HAVE a brother at all!" Brace yourself to hear that and don't get upset at him for expressing those kinds of feelings.
We made a big deal about "OK, this baby is going to be YOUR brother. Mom and Dad will help you take care of him and do things like diaper changes and feeding, but he's YOUR brother, OK?" And we talked about what kinds of things he'll be able to teach his baby brother to do when he gets bigger. He helped do some of the shopping for the baby and picked out some baby clothes (a wild tie dye!)
And after the baby was born, the first time the older son came to visit (in the hospital), I figured he wanted mostly to see mom, so I made sure I wasn't holding the baby when he came in the room. He was able to come in, get a mom hug, and I waited until HE asked "Where is he? Let me see him!" (I didn't have to wait long.)
When the big brother kept the baby from rolling off the bed one time, we made a big deal about "saving his brother" and being a hero. A REAL hero, not just a superhero that's in the cartoons. I think we even immediately called up the grandparents to tell them all about it. Boy, he really loved that! He kept a super sharp eye on his brother for weeks, hoping to keep him from taking a tumble.
And we still point out to my older son (now 5) that the baby (now a toddler) loves HIM best of all - more than Daddy, more than Mommy - more than ANYONE. (Actually, I think my son and I tie for first). I ask "Who does Rob love more than anyone else?" (older son smiles points to his chest.) "And who is Rob's hero?" (again, points to his chest.)