I think you came across in a way you didn't intend in your OP. But how else can people interpret your post when presented the way it was?
As it is, you shouldn't force/trick your DH into having an extra child (not suggesting you would) so that only reasonable thing you can do is have a sensible chat with him about it all. but if he doesn't want another child, you must respect his wishes.
This isn't the same thing as someone saying "I'd love kids, defintiely" and you partly basing a relationship on that, then that person going "Ugh, I never wanted kids, stop asking". That scenario is full on lying and changing your mind, and being deceitful etc.
However, your situation is he wasn't bothered how many he had, until now. It doesn't sound like he has "Changed his mind" it sounds like, he's now just decided he only wants three.
So yes, you will have to make a compromise in this if he genuinely doesn't want a fourth child.
Would it be fair of him to put pressure on you to have an extra child that you didn't want? Would it be fair for him to resent you not wanting an extra child?