Hello G.,
Just a few months ago or so, I was in your exact same position. I am currently pregnant with my second, to be due around March 7 while my first just turned four years old a couple of weeks ago. I felt so bad about neglecting my son or leaving him out of anything that I actually went into a slight depression. Now, I've made sure my son is included in absolutely everything that has to do with the baby (which is another little boy), including coming to every single appointment and helping us put things together (like the crib and such). I let him feel his baby brother kick and even let him help us pick out a good name for the little guy (which happened recently). He is actually so excited about having a baby around, that he talks about giving the baby his cars and he even gave him his little pillow that he used to use when he was younger (it has an allergy protector case over it, which I do with all of our pillows). He understands that mommy is having a baby and the baby is inside mommy's tummy. At first, he was a little scared when he heard the baby's heartbeat, but when we came home, (I'm in nursing school), I took out my stethoscope and let him listen to his and my heartbeat so he could understand. I've just made sure I've included him in absolutely everything that has to do with us and the baby! He now is okay with the heartbeat and actually enjoys going to the "baby's doctor" to see or hear his baby brother.
Also, keep in mind that since you are newly pregnant, your emotions are going to go wild due to all the hormones and you are worried about the most dearest thing in your life, your first born. Once you hit your second trimester, those feelings seem to level out a bit and both you and your child will be way more understanding and accepting of the fact that a second child is on the way. Also, know that if you are worried and upset, your child will surely sense that, even if you don't allow it to show. So, just know that it's all perfectly normal and go ahead and look at my previous post to see what I wrote and what others responded.
What area do you live in? Since we have children in the same age group, who knows, maybe we can meet up and have playdates with the kids or become friends.
I hope my advice has helped you at least a little and please, feel free to write me and ask me any further questions.
Have a soon to be Merry Christmas!!!
P.S. with your first born being four years old, he'll be the best little helper around there is to be! I think four years is the perfect gap between siblings! My son's at the age to being so helpful, but not to the point of forming his own opinions and NOT wanting to help. He loves helping mommy and daddy (and soon to be baby)!