I voted 4", although a good case can be made for 3". It really depends on just how much CCW ergonomics factors into it, vs increased sight radius & velocity.
4" really pushes the envelope for concealability (at least for me), especially with larger frame revolvers. But the extra inch of length is worth it for everything else, and that is what made up my mind. I'll just keep a "dedicated" CCW gun around for that niche.