Yes. Far better the OP leave her husband/try her luck on the singles market and start a second family rather than accept life with three children. I'm sure this is in the best interest of her existing children.
Yup. Exactly. I find the amount of people who are really unquestioningly siding with the OP a little worrying...I mean. You have 3. You've never really provided a good reason, except having a big family of your own, why you desperately want and crave a 4th, except for a false comparison to your own family. It's easy to think "oh it's only one child," but I think you are being a bit naive. Your 4th child could be twins, could have special needs, or could just be bloody hard work. I personally think it would almost be easier to raise a child like that on your own than with a totally disengaged father. Not giving you any ideas. I just think the blasé whiff of "well, I do what I want" sounds naive and unfair.
Because...you've never known anybody who regrets having children? Perhaps because that is completely taboo to say in society. EVERYBODY would judge a parent who was "heart-on-sleeve" about wishing they'd never had their child. I would imagine the bloodbath would be something tantamount to a person who had cold-bloodedly killed their children.
They do exist, and are more common than you might have thought but, taking aside the really evil/monstrous ones, the vast majority of them just get along and make the best of it because they appreciate it's not their child's fault. And, frankly, OP, if you love being a mum and you're absolutely obsessed with having more children, you're probably not the kind of person who they'd admit it to. That's not a criticism, it's a lovely thing to love children.
Your disappointment is not unreasonable. Your response to your disappointment is, in my opinion, totally unreasonable, and I'm beginning to see a bit (as cruel as you might have found it) why your DH put it off, because it doesn't really sound to me like he ever wanted 4, or not once you'd had 3 (!), which is not unreasonable of him either. He might've liked the idea of a big family and once he got there been like, "oomph, okay." As unfair as it might seem, even if you do most of the work (and I'm not trying to piss on you, OP, but try to take a reasonable account of what he DOES do, minimal though it might be, because, particularly with many children, every little helps), and ask yourself if you would find having many children as pleasurable without him.