The Fourth Void

We never had a baby that wasn't edging the last baby into toddlerhood just a little sooner than they liked. This is the longest span of time we've not had a newborn since I married my husband. I am simultaneously grateful for this space and filled with an ache to fill it. It is strange to feel the pull of something so strongly, in all the little things of our daily life I see so clearly another child. I long for the chance to relish and enjoy the smallness of infancy that disappears so quickly that we never really seemed to be able to notice because things were so busy.
I feel the fourth void like a missing child. I can so clearly see our family with four kids that it sometimes feels like something is missing, or I am leaving one behind.
I want to barter and bargain by saying I will do it as much on my own as I can, I will sleep less and not care. I will do as much of the house work and child work as I can, give as much space as I can. These won't tip the balance or make the difference. I know our life, we are both all in.  I want to promise alone time. I want to promise an addition to our house that will give us much needed space. However, additions are years off and alone time in one and two hour increments is not really enough. I would do anything to have this baby I feel is the perfect ending to this space and time in our family. The problem is, anything isn't in my power to give.
I feel like I will always regret not filling this fourth little void,  I am afraid I won't stop wanting this. I am afraid my heart will hold this sadness and I won't get over it. I am afraid it will make me angry and snippy, and short even though I see the opposing side too.
Where do you go when there is no compromise, there is no such thing as half a child. There is no love loss here on the issue, I see the tenderness in my husband as he watches me grieve through this. I also see his feelings of finality that he cannot change. So we sit at an impasse, happily married and in sync in every issue but this one. Years have drifted over this impasse, and still we wait for an answer we can't seem to find. What is more regrettable, the baby never had or the baby hard fought for? I truly don't know anymore