I thought I wanted 3 dc, but dc2 is now 14 months and sadly I'm coming to terms with the idea we are done. For us, it's a combination of things, but mainly finances, we are going to struggle to give 2 the lifestyle we want, 3 (or more) would make that impossible. Emotionally, I already feel dragged in two directions, a 3rd would make that worse and I'm not sure how you give them all a good deal of attention and 1-1 time with 3+.
What I do struggle with is not so much not having another older child, but knowing I'm never going to be pregnant again, never having name conversations, never having a baby again. So actually acknowledging it's the pregnancy and baby stage id like again makes it easier, because a 3rd would only quench that desire for a relatively short period, then they'd be a toddler and I'd still have to deal with the "never again" feeling.
I know a 3rd would make my existing dcs lives harder, they would have less opportunities and less time with me/DH. That has helped focus my thoughts because it's a selfish thing I want, however if you are in a better position financially that might not be a consideration for you.
It has also helped that I recently spent time with a friend who has a 6month old dc3, her older dcs are playing up like mad trying to get her attention and she's finding 3 a lot harder than 2.
Ultimately, if your DH doesn't want more and you want to stay with him, that's it, you're done. An unwanted child would be unfair on everyone, including the child.
Think about why you want a 3rd/4th, if those babies would make you feel done, or if it's the baby stage you want, in which case, another one or two children won't stop that.