It is a pity that all the helpful advice applicable to your situation, I could give,  is based on the past.
When you were not expecting a baby with your girlfriend.
I would have told you to walk away, leave.
You have or had a choice to be a step parent.
Your stepson did not have a choice in being a step child.
He is at the mercy of the decisions made by the adults in his life.

I can understand not being able to love a child that is not your own.
I can not understand - though honestly put - your immaturity with regard to someone else's generally well behaved child.
You say you could not leave because you loved your GF too much.
Your love does not extend to her child and a double standard you know you have about caring for the children.

You do not want to be a step parent.
Yet you are.
Find some way to deal with your attitude because if you continue, your own child will one day be a step child.
No decent woman worth her salt will allow a partner to treat her children differently.
One child may be yours, both are hers.
See the problem?

If you did not want the child to call you Daddy, you should have shut it down right at the beginning.
You did not.
You are the cause of your own issues.
So, when these issues and your dual attitude to her children becomes cause for your seperation, please hope that the person who is stepfather to your child, treats your child better.

In future, take control of your reproductive rights and capabilities too.
If you want a child with someone, have one.
If not, YOU wrap it up.
Unless you want a string of children with different women.

You are 26.
This relationship may or may not last either.
Lets hope your prospective future girlfriend or wife doesn't have the same attitude to stepchildren that you have,

You may have a new partner and your current girlfriend may end up with a new partner.
Your child with current GF can end up with TWO stepparents.
How would you feel if BOTH those two people wish your child with GF did not exist, or made your child feel neglected?

It is more than YOUR feelings at stake here.
Grow up.

I agree with EVERYTHING Letti R has said. 

I will add something else. I know you've been around this child for a long time, but it is not too late to learn to love this child. You simply have to act in all ways as if you did until you do. It will take a lot of maturity and a big leap. 

You are setting your own child up for a lot of heartache if you don't. By treating the kids differently you will create a rivalry and animosity that your child to be doesn't deserve. A sibling can be your greatest ally - but not if a parent has been sabotaging the relationship from the get go. 

At least you have enough foresight to know you’re going to mess this child up emotionally? Shrugs. Perhaps get yourself into counseling, because you dislike a 4 year old for basically no reason other than he doesn’t share your DNA.

There are a lot of people on this site that despise their step kids becasue they are totally a$$holes... but based on what you have said here... you are the a$$hole. This kid, a good, kind, loving kid- sees you as his dad. You are the only dad he knows. You are about to be a biological dad too. Pull your head out of your own butt, stop getting in your own way and be the best dad you can be to both of these babies. If you can't do that, then leave. All 3 of them- SS, your bio and their mom deserve someone who will be there for them all. There is a man out there who is a good enough man to step up and do that. 

You sound like a 26 year old guy who should never have children because you are not ready to be a father. You should consider taking some parenting classes. Talk with a therapist about your feelings about this 4 year old boy. You could mess up this kids life if you dislike him. From what you wrote this child sounds like a loving little boy. Everything you wrote about in your blog comes right back to you. You chose to have a relationship with this GF and her child. Now you are bringing another child into the mix. You made these choices. You need to get your act together and man up. If you don't want to be a step parent than leave.

A mammalian visceral reaction.

For some men, me included, there can be what I will describe as a visceral reaction to the spawn of another man in your home.  IMHO this is by all indicators a visceral mammalian reaction.  Take lions for example, when a new male takes over a pride usually the first thing he does is kill the young cubs of his predecessor.  There is some lion physiology involved in this since the females will immediately go into heat when their cubs are killed so that the new male can propagate his downline.

Fortunately... we are not lions and have a brain that we can engage to resolve this issue.  No young child should be either abused, mistreated or rejected over adult issues.

I had this same reaction with my SS when my DW and I started dating (he was 15mos old) and ultimately married the week before he turned 2yo. I knew that this amazing woman was who I wanted to spend my life with and to do that I had to engage as my SS's father. So that is what I did. I held his hand when we walked or put him on my shoulders, I took him to parks to play, we chased ducks, searched for fossils, went on "Johnny Quest" adventures, I read him bedtime stories, he sat in my lap while we watched Disney Movies, when I was working on my MBA at night (online) he "studied" with me, etc, etc, etc.....  I did homework, wrote papers, did research and he read Dr. Seuss books.  Our activities were often the three of us together but just as often it was just he and I when his mom was in school, studying or working late.  Those actions of love built the feelings.  I truly and quickly grew to love the kid.  He is my son. 

My SS-25 was a great kid, mellow, smart, etc.... The issue was me and not him.

So, I raised him as my own and eventually he asked me to adopt him (he was 22).

Give it a try.  It works. 

This is exactly what happened to my SS, and it's really hard to watch.  His mother insisted he call his step father "dad" and always pushed them to be closer.  Well it worked, my SS and his step father seemed to have such a close relationship it honestly made me jealous sometimes that I couldn't get closer to SS.

Flash forward a few years and BM and her husband had a kid, and now my SS's step father clearly plays favorites with his bio son.  I think it wouldn't be such a big deal if he had been clear from the get-go that he wasn't going to be "dad," but he didn't and now the change in their relationship and the difference between SS and his half brother is seriously noticeable.  Let me tell you that SS now has very little respect for his step father and as he gets older, he challenges him more and more.  Believe me, the teenage years in that house are going to be hell.

Unfortunately, you have already established a relationship with your SS and I don't think you can change it, but what you can change is your attitude.  You may not love him, but you can make sure that you always treat him and your future baby equally.  Fake it til you make it.

I agree with everyone who has stated that you need to fake it till you make it, this boy sounds like a good kid and you want him to continue to be a good kid, pushing him away making him feel second place etc will not accomplish that.  Without legal paperwork you have taken on a commitment, the raising of a child, it sounds like the boy already loves you and if you allow yourself to feel that love you won't be able to not return it.  If this is something you are not up for , leave now, that boy deserves a chance, I can't promise he won't grow up and make bad choices that will hurt and anger you but he still deserves a man to be the father he needs and sounds like he wants, so either man up do the right thing by your GF and her son or hit the bricks.  Without a biodad in the pic your road will be sooooo much easier with him and could be one of the most rewarding things you ever do.

I wish I had met my sd when she was at the age, that way we would have a better bond than we have now. I met her at 8. Birth to 5 years are some of the most important years of a child’s life as far as bonding goes. You are creating a life with this women. You might marry her one day. You don’t have to love him but fake it til you make it and be grateful that the kid likes you and try to form some kind of bond now or you will regret it in 10 years when he is a little shit and hates your guts.  At least you met him at the age where you can be a good influence on future behavior, some of us can’t say the same.