I think you're confusing the 4" barrel length with the model #. The 686-4, which is out of production as others indicated, came in a number of barrel lengths. Current model is at least a -6 and may be up to a -8 (mine is about a year old and is a -6, but mine is a 7-shot "plus" model). S&W website does not show the "dash" number of the model.

If you want to neuter the IL, a forum member (who has contributed to this thread) sells "The Plug" which fills the hole nicely. Installed one on my 686+ when I neutered the lock.