We only have a 3 bed and 2 share already, dh works all the hours he can and we're not very well off and at 40 I imagine it will destroy what's left of my body.
Congratulations, you have choices.
I'd be wanting to know what your DH means when he expresses delight and promised to work 'even harder' , given that you describe him as already working all the hours. That doesn't sound as if he has a very firm grip on the reality of parenting a fourth child, even before you factor in the risks of special and additional needs.
It sounds as if you already have a lovely family. Why would you stretch your (very finite) resources to cover another, unplanned addition?
What would be tbe benefit ?
My darkest fear would that your DH was keen to keep you away from making a career for yourself outside the home.
There would be no question in my mind what I would do in your place: I would be thanking providence that I had decent, legal and safe choices.
I do worry he will resent me if I go through with the termination
Your DH has expressed his opinion and left it as your choice. Do you really not trust that he really means what he says? Has he given you any reason to not trust what he says?