Well, thinking of 'could've', 'should've', and 'would've', this SHOULD always be a decision that BOTH partners AGREE upon before 'biting the bullet', so-to-speak. It's too late for that, but he must feel VERY strongly about NOT having more children or he WOULDn't have done this.
You can only do what you can do . . . pray that your heart will be fulfilled, softened, and not embittered. LOVE your husband and the children that you HAVE. Don't miss out on one happy memory-in-the-making with the people/children who DO exist in your life by fretting over the 'ones that COULD've been'.
Whatsoever things ARE pure, honest, of good report, etc -- think on THESE things.
I'm not trying to invalidate your FEELINGS (we can't help our FEELINGS, and 'feelings', per se, are NEVER evil), but we all have to decide what to DO with our feelings and channel our emotions, mentality, spirituality, etc. in the right directions.
And, by all means, meet with SOMEONE (even if only other girlfriends) to talk this through. As women, I think the most important thing IS to have our 'feelings' validated!
May God bless and comfort you through this!
P.S. We also have 4 children (all grown and gone) born over a span of over 11 years, and we STILL waited until the youngest was 6 before deciding to make a 'permanent' decision (Thankfully, HE 'bit the bullet', as it was a somewhat easier procedure for him at that point than for me!) Now we have grandchildren (3), and we STILL may Foster-Parent at some point. Life goes on and there's much to be done . . .