I'll give you a REader's Digest answer. Recessed means that the back of the cylinder is not flat. There is a hole in the back of the cylinder that the case rims fit down into, so that when you put the cases in, the top of the case is now flat w/the rest of the cylinder. IMO, recessed is not a plus, except to somebody that thinks that is hot-stuff.
Pinned is no big deal either, I think, that all bbls since then are pinned. The bbl has a flat spot on it, on top. They torque the bbl on, and put a pin through, so that it won't unscrew. I'm pretty sure that all S&Ws since then have it and I've never heard of one loosening up...except for my match gun, which has over 100K on it.
I know of no valid reason why a 19-4 would be more valuable than any other model 19. In fact, I'd rather have a model 66 or a 686 (stainless version and a stainless version on a slightly bigger frame). The most desirable version is determined by its use. A k38 is only desirable as a collector, they are hard to shoot and a "whippy"
What to look for: Pull the trigger, slowly, before the hammer falls the cylinder should lock into place. S&W considers acceptable if it locks when the hammer falls, but not me. Some folks turn the gun upside down to to this (it locks underneath the cylinder, turning it put is on top where they can see it). You should pull it six X, it should lock correclty on all 6 cylinders.
Dryfire it (it will NOT harm the gun), how does the trigger pull feel?
Lock the hammer back, the cylinder locked (it better be locked into place), jiggle it side to side, it should be tight, w/just a slight jiggle. push forward on the hammer (w/it still cocked) it should not go forward. Same thing, now push the cylinder forwards and back, there should be movement, but not a sloppy fit.
pull the hammer back slightly, spin the cylinder. Are there high spots? The cylinder should spin evenly for a revolution or two (maybe much more, if it has been tuned). The thing to look for, is it out of round and does it drag (as if it is too tight, front to rear). I once bought an older M14, and it dragged like crazy, this is easily fixable. IN '08 I was about 8th, overall, w/that gun at the nationals. NOw, if the cylinder is out of alignment (as in not concentric w/the bore), this is a harder fix...but still do-able.
Open the cylinder, look on the back of the frame, are there images of the cartrdiges cases in the bluing? When fired, the cases will recoil against the frame and wear off the bluing, after thousands and thousands of rounds. Look at the slot on the cylinder, the bolt (I think that is the name) sticks into the slot and locks it, but it drags on the cylinder as it is rotated and wears the bluing off...how long does it look like it has been dragging on the cylinder? If you hardly see a line, it has hardly been shot...if it is real clear, then you are also going to see where the case heads have worn the bluing off of the frame,,,which is no big deal, if everything else is w/in specs, or the price is right.
Now, nothing on a S&W revolver wears out, shot of 100,000 rounds. Sometimes they are not fitted correctly and need work, but that is about it. W/.38 ammo, the bbls last forever...if CLEANED CORRECTLY. The springs are a replacement item and the only thing the ever wears out is the hand (which a smith can change) and/or the star. A star wears out at over 100K, if not more.
S&W will fix all of the above problems, sometimes for free. I can give you the names of several smiths that can work on your gun that are trustworthy and sometimes expensive.
Now, cost...anything under $350 is a buyers price. For a pristine gun, you can expect to pay upto $450. New ones are about 550 (I think). Avoid a new gun as they have MIM parts in them and can not be tuned. If all you want to do is change the springs in it, then they are just fine. A dealer should be able to get you some security guard turn-ins for under $300 and they will possibly have some problems w/them...but they are a very good starting point, if you want them to be tuned up.