I usually give people two in a short period of time, because I have known many people who, while it was their second wedding, it may have been their second spouse's first wedding, and part of me thinks that the second spouse deserves a "big" wedding if that's what they want.
Your DW is in a bad position because it's her best friend and I'd want to be there for my best friend, even if I didn't think getting married was the best idea for her. Your DW is kind of stuck between a rock and a hard place.
My oldest friend has been married 3 times...the first when we were in HS to the father of her child, which was obviously a mistake. The second was to a great guy to whom she was married for 17 years. The third, and current, is to her life partner. They had a commitment ceremony about 3 years ago that I also attended. My friend has led a very colorful life! Now, these three weddings took place over the course of 30 years, so it is a it different than 4 weddings in 10 years.
These things are hard, but as her DH, I think you should go along on this one. I am sure she's done things for you which she hasn't necessarily agreed with, but has gone along with them for your sake.