I have one felony. Robbery I did 4yrs for my charge is there any way that I could pose a problem to her gaining primary custody of her 1yr old son?

Anything is possible. What was involved to elevate it to a felony?
Did you use a gun?

No I did not use a gun. A small blade.

Ok, you say "lose" custody, and then you say "gaining" those are two different things, in one she already has it and would have it taken away, in the other she does not have it and is trying to get it.

First if the other parent lives close and wants joint custody, there may be no reason that the other parent could not get it, ( with or without you in the picture)

But yes the other parent could show you as a possible danger to the children, a "known" felon who used a weapon in a felony. They are permitted to bring this up in court.
I don't see where it should make a difference if the mother is a good mother, but you never know what can happen in court

I was convicted of robbery with a dangerous weapon. The weapon was a small knife. I took a plea for 4 yrs. I'm on probahation and am working full time doing well for myself. I live in North Carolina. My girlfriend has a 1yr old son, we don't live together. She just got a divorcr & is getting ready to start a custody battle with her ex-husband. I want to know can I actually be a main cause for her not to have primary custody of her son? If so how can a judge take a child from his "Good Mother"?

How would I pose an immediate danger to the child as a changed man that works and has made his way straight.

How would you prove that you are a changed man? You work, that's one way. Time passing with no arrests is another. Not so much as a parking ticket is a third.
We can't know what the court would decide when this subject has so many variables, such as how much time passed is enough, or how much protest the father of the child makes, or what the mother's history has been, and what the current custody arrangement is. If you can afford a consultation with a family law lawyer in person with both of you, I would suggest it.

When you say changed man, in what way have you changed? By working? Thank just means you are able to work and it probably is condition of your parole, so I don't necessarily see a change there.
Time and your actions will tell, TALK IS CHEAP. I can say I am a recovered alcoholic, but in reality I am never recovered.. I work at it one day at a time. How long do you need to be straight to be a changed man? Who knows, But I have seen hundreds if not thousands of men/women say "I'm a changed person" and they are, just until the fear wears off or they are cleared from the courts again, and then BAM!! They go back to the way they are most comfortable with in themselves.
Living a straight life is not easy and has a lot of ups and downs. I hope you have changed for the better, live it right for today, after all today is all we have.

... What was involved to elevate it to a felony?
Did you use a gun?

Normally robbery, "elevated" or not, would be a felony.

Normally robbery, "elevated" or not, would be a felony.

What you have to understand is that in most cases they lower charges if things go smoothly. So committing a crime that cause full charges to be pressed is elevation in the nature of the business.