" I wasn't that bothered about DC1's arrival, or DC2, they just arrived without effort or planning. D3 was planned but again effortless. Dc4 was the most longed for child in the world."
So you've no idea what it's like to struggle to have any children at all, not a clue about what it's like to be worried about giving your child a sibling.
But somehow you've worked out that it is just as bad not to be able to have a 4th child?
Insensitive, self-indulgent shite.
If you really couldn't take any solace from your healthy, living 3 children, I feel very sorry for them.
As for "most longed for child in the world"?
The OP isn't suffering from infertility - she has 3 children and her husband is done. Comparing that to a woman struggling to become a mother is crass in the extreme.
OP - I think YABU, but you feel how you feel.
I am pg with DC3 now and the question of DC4 is unanswered. I think DH is probably done, so I can well imagine myself in your situation in a few years.
A day or two of "what might have been" melancholy once a month seems a reasonable self-indulgence, but I hope you are not letting it spoil the good things you do have :)