Sounds like you already have some questions on how this will work out, and you are right a child will not stay hidden for long. A more pressing point do you really want the father of your child hiding that fact? A even bigger issue is what happens if his wife finds out and decides she does not want him to have anything to do with you or the child from now on? The fact is it would not be the first time a man has abandoned a child simply because his wife made him do so. Do you really want to set your child up for that kind of rejection?
What you are doing is not right.......but you do not need me to tell you that. You already know this is not fair to yourself or to his wife. You are a women who deserves to have a guy who puts you above everything else and wants you only. He made a commitment and a promise to his wife, and is breaking it. Which is wrong, but if he is unhappy in his marriage then he must get out of it. If he is truly getting a divorce then this would be diffrent...........but you know as well as I do that if you are looking to divorce someone you do not stay with them.
The relationship is troubled to begin with, adding a child is not going to help matters. If you really love him and he really loves you then it is time to take a stand and let him know that while you may want to be in a relationship children need to wait until he has decided what and who he wants. Now I would not advise a child right now based solely on the fact that you have only known him 4 months.....that is not long enough to build a lasting relationship let alone time to build a family.
It sounds like you already know that relationships do not last........do you want your child growing up with a father in a different home and having to explain to your child that you made the choice to have him or her with a married man? This is going to be a valid question your child is going to ask someday..........do you really want to answer that?
Everything you do now will effect your child, the last thing you want is to have to raise a child alone or with a man who has so little respect for you or his own child that he would even begin to place you in this position. If he really wants a child with you then why would he want to put his own child in this position? The fact is things happen sometimes......but any man who would intentionally set out to put his child in a situation such as this is not a very good man nor would he likely be a very good father.
What I would suggest is you sit down with him and decide what you both want. If he really wants to be with you then he needs to end it with his wife. I do not condone this and I really think he is quite shallow to do this to you and his wife but the reality is it is better for him to leave her now if he wants to be with you. Continuing to be in this type of relationship is not good for you.
You deserve a man who is going to be with you and you only. Ideally you would break this off, but I understand this is not always as easy done. Either way continuing this relationship and adding a child to it is simply not a good idea for anyone....least of all your child.