Frankie2blue,
It's somewhat academic to me.
If you are looking at a shelf full of M686's in the right configuration, all with a different engineering change, then I'd go for a -4 if you can find one first, just because it has features that appeal to me from an aesthetic and functional level (there's nothing non-functional about a -5, just less pleasing from an aesthetic point of view to my traditional eye).
Now, if I was looking for a hell stout revolver in 357 Mag with all sorts of K frame models on that same shelf and one M686-5, it would be a tough call since the M686 will give you much more flexibility than the K frames and longer service life with stout loads.
There is no "one answer" for everyone, only your requirements will matter when it comes to spending your money.
If you are looking to put your money into something that will ensure value/worth growth by itself, then I wouldn't be looking at M686's for that.
These are fantastic workhorses, and some are collectible from a scarcity standpoint, but for just plain robust shooting the -4 and -5 will probably rate side by side over the longterm.