I certainly understand your longing. I hope your dreams come true. Four seems like a magical number to me.
One thing that seems to help my husband make a sacrifice for me is that I am pretty supportive of him, even when he has a harebrained scheme. He usually, but not always, comes to his senses before too long. So after he has given up the idea of moving out of state to become a crop duster, he remembers that I said "Wow, how cool! Tell me more. Maybe we could live in a camper at the airport.", while I was trying not to have a panic attack. Would embracing his ideas of retirement help in anyway? I am like you in that I have often said, "In for a penny, in for a pound." Is retirement and having one more kiddo necessarily mutually exclusive? I don't think a parent has to provide a four/five year college education. Sometimes those who need to be most resourceful end up having very resourceful kiddos. You strike me as extremely resourceful
What about exclusively breastfeeding? Is that getting your husband down? My husband calls EBF the marital romance kryptonite. He is supportive, but oh wow, has he had to take a back seat in many regards due to somebody always being attached to my body. Then when there is nobody attached, I just seem to want my whole body all to myself and want some space. EBF has a limited timeframe, and you may see a more open husband after your little birthday girl weans.
I think he will get there eventually. I really do.