J.B.
I agree with being grateful for the children you have. Enjoy them wholeheartedly. Enjoy every minute you have with them. Be happy that you have three. Many women out there aren't even lucky enough to have one.
And giving him time is a good idea, too. Those diapers and formula (if you use it) are expensive stuff. I'm sure adding ANOTHER one to the bunch may be a little stressful for him (especially if you're not working). Again, enjoy what you have and know that if it's meant to be, it's meant to be.
Lastly, if you wait some time and still feel you are unfulfilled and HE is still stuck on "NO MORE", than try volunteering. Get involved in a "big brother, big sister" program. Or get in the mindset that you will always be the mom that goes on fieldtrips and things for your children when they're in school. Maybe you could volunteer at your hospital and help hold the "risk" infants. That could be all you're after anyway - is that new baby thing. I know I get like that, too. I just LOVE that new baby.
Many Blessings to you!