Can anyone explain what changes S&W made between the original 686 and the 686-4? I have read elsewhere that the 686-4 had all the desirable upgrades...what would those be?

Welcome to the forum.
The M686 started production in 1980. In 1986 the M686-1 had the radius stud package and floating hand. In 1987 S&W recalled both these models to change the hammer nose bushing and associated parts. If the modification has been done there should be a M stamped over Mod686 on the frame.
The M686-2 started in 1987 with the same hammer nose bushing change.
The M686-3 started in 1988 with a new yoke retention system.
And the M686-4 started in 1993 with a changed rear sight leaf, drill and tap frame (for scope mount,I think), change extractor and introduce Hogue grips.
Now exactly what all that means I'm not sure but that's what the book says. If you need to know maybe one of the other members can explain it.
Again welcome to the forum.

Just to add a couple notes.
The 686-4 is also available with the 7-shot cylinder. (The first I think).
The 686-5 marks the change to the flat hammer nose and new CNC frame, so the 686-4 is the last of the hammer mounted firing pin on the old familiar frame.

Great info. Thanks for your help. For a minute I thought you were a walking encyclopedia! 8^)

The SCSW (Standard Catalog of Smith and Wesson) is a good investment.

I don't think it's as much as the -4 is better than the earlier models, as it is the last to use forged parts and the hammer mounted firing pin. A lot of people feel the MIM and frame mounted pin was a cheaper way of doing things. Right or wrong it has made the -4s' probably the most desirable.

I’ll take any 686. I own only 3 so far. I’m down to 7 S&W revolvers.

I have quite a few 686's and the two that I like the best is an old 6" no dash with the M stamp that has been shot to *****, and a newer 5" Pro.