I went though the same feelings after my third child. I have 4 and there is a span of 15 years between my oldest and my youngest. I have never regretted making the decision to have my 4th child. From the minute she was placed in my arms, I just knew my family was complete.
After the birth of my youngest, I would still feel the occasional twinges sometimes when I held someone else's newborn, but never the longing I had felt before her birth. There has not been a moment in the 14 years of her life that I felt I made the wrong decision.
Like another poster before me mentioned, I have a close friend who was struggling with the same decision at the same time I was. She still regrets choosing not to have that last child.
Good luck with your decision. I know it's a tough one.
Oh, when my first grandchild was born, my youngest daughter was only 5. Holidays at my house are never dull!