Am I too old to have another baby?

I will be 40 on Sept. 4th. I have a 5 1/2 year old daughter, a 4 year old daughter, and a 20 month old son. I am a stay at home mom (former 4th grade teacher), my husband is self employed, so finances are not a problem. Here is the problem: I want another baby, but am I pushing my luck? I am so blessed with my family, So why won't this wanting another baby feeling go away? Oh yea!! I forgot to tell you the most important part- my husband had a vasectomy a year ago, and is considering a reversal because we feel we made a mistake to begin with. I have prayed so much about this, but need advice form other women I realize only my husband and I can make this decision, but it would be nice to get some opinions Thanks so much!