5" is the length most often missing. 4" guns are great, but could benefit from a tad more velocity and sight radius. 6" are nice range toys, but getting a bit long for general duty as a holster carried gun. 5" is perfect.
Strikes me as odd that the most common barrel length for full size autoloaders is so scarce on wheelguns, too. Try to think of a full size auto that doesn't have a barrel between 4.5"-5.25"......