A.J.
I understand how you feel, and if you had only one or two, I'd tell you to keep pressing. And I'm a huge supporter of huge families, most of my relatives have at least 7 kids!!! But your husband is your partner, I'm guessing he supports you if you are a sahm, and with this many kids, and so much happiness, try to be grateful and respect him.
However, if you want a secret weapon: Say, "OK, honey, You know how I feel, you know my age, I so badly want another baby and I know it would be amazing, but I realize you are supporting all of us, and I have to respect you. You're the most amazing dad in the world, and I love you with all my heart, so I'm not going to bother you about it anymore, even though I'm sad inside."
And then DON'T BUG HIM ANYMORE. Treat him like a king for the next 6 months with no pressure.....and he just may get generous with you. Or not.
Either way, you're very blessed. I have 3 and definitely wish I started younger and had more...but I have decided to feel blessed and fulfilled. My husband doesn't want more, and it's it's not like I have idle time to fret about it while he always travels. I'd LOVE another son to balance out my 2 girls 1 boy, but some people don't have ANY kids and really want some. You are VERY blessed.