Oh I absolutely agree with the 'he wants a child, I don't' thing. It isn't fair on him if there isn't a desire to have more DC, but in OP's defense (and as cack-handed as it sounds) what does he expect from a woman in her 40's with 4 kids already?
Surely the writing IS on the wall there that it's highly likely that she won't want anymore.
If that is the case, the kindest thing is to let him move on and find someone with whom he has a greater chance of having a family of his own.
I think him demanding that she try, while not apparently taking that much interest in getting his hands dirty, and moreover threatening her with being dumped is the key issue here.
He wants her to 'prove' her love for him by ttc a 5th child.
When she's done that' I think we'll all guarantee that there will be other tests. There always are when it starts like this.
spirited he's threatened that unless you try to have his child, he's off.
Normal men, in these circumstances, with more than a single IQ point don't do that.
You have to see that this guy isn't the kind of guy you should ever allow to adopt your DC?
As shit as my ex is, my DS is mine, and mine alone, I'd never consider having another man adopt him.
I think, in order for you to regain power in this relationship that you call his bluff, tell him you're not going to try and that he's welcome to leave. Don't even blink when you do it. You are more than strong enough to follow this through.
There are distinct advantages of NOT being 30, and one of them is being able to run rings around those that are.