I have four and like you my family just didn't feel complete when we only had three. My advice is to try to at least get your husband's support before you go ahead with number four. Having a large family keeps you busier and all things added up, the pregnancy could be a stressful time for you, so it is important to have his support. But then, granted your kids are more spread out than mine, so maybe they will be able to be a bit more helpful.
I think (from other people's stories) that if you keep bringing up the topic (not to the point where you are nagging) but just as an idea, your DH might become more used to the idea and accepting of it. Do you have any friends or relatives with newborns? Maybe if they brought their babies around to your home while your hubby was there and he could see how cute they were and how happy you were around them that might make him think differently too... or get together with people with larger families and talk about it with them - the pros and cons.. I think most people ONCE they have reached a larger sized family will see the pros over the cons and not have much bad stuff to say about it. I can't say anything bad about it even though obviously it's not perfect - it's perfect for us.
Anyway, just some ideas...
Good luck!