I have the 3" version. I have seen the 4" guns and I think they are less seen than the 3 inchers. Whether or not that would make them more valueable is open to question. With this type of limited production it is probably more as to who wants it? I paid a little more than I probably should have for my 3", but 1. I love 3" Smiths, and 2. That particular gun was exactly what I wanted, L frame 7 shot carry gun. Has a great action and with a couple of HKS speedloaders I have plenty of firepower. Usually though I feel that if once cannot settle a SD issue with 5-7 shots of decent 38/357 ammo, you need a shotgun.
Packing for duty (which I do) the 681 is a really nice package, I can train with the full power 357 ammo (couldn't with the M66) all day if I want to, and not worry about forcing cone issues. It also is a nice package for full power mag ammo, nice weight to absorb recoil and enable rapid repeat shots. It is a thoroughly modern wheel gun and will hold its own against any tupperware bottom feeders.
I would value mine at about $850, if I really wanted the 4" or needed it, maybe $50 more.