The frames & slides have Box-type rails, instead of the older step-type rails. Thicker.
The barrels received continual revisions here and there. A slight change in the chamber taper in the 59XXTSW's, thicker barrel hoods, different machining in the feedramps, chamber mouths, roll-over points, etc.
They started machining the frames to allow changing mag catches for ambi use in some double column frames (you can see the "reversed" machining on the right side of the frame allowing the mag catch to be flipped and installed), but the .45 frames still required the virtually unknown (even inside the company) LH mag catch to make it an ambi setup.
The inside of the slides received some better finish and machining to relieve potential stress spots. The increased use of CNC machines, closer tolerances and better precision all helped.
The assemblies and parts, while mostly the same as those used in the standard 3rd gen guns, also benefited from improvements, revisions and better tolerances.
While the delayed unlocking would help with reducing the felt recoil experienced by the shooter, and tighter frame/slide tolerances (Box rails) would promote better inherent lockup and accuracy, I'd be surprised if most average shooters would really notice the difference.
I did like the loaded chamber inspection port incorporated in the later TSW's. I wish my original 4513TSW barrel hood had come machined with it.
Also, the later production TSW's (and the 4040PD, oddly enough) received the extra pair of machining steps necessary to allow them to accept the spring-loaded, decock-only assembly ... but good luck trying to find those assemblies at the moment. Last I heard, S&W wasn't ordering anymore of them for new commercial sales parts inventory (keeping some for repair-only use, as some LE agency customers have ordered their guns equipped with them). Maybe later on, but right now their money and attention is being focused elsewhere.
The only thing I disliked about the TSW's was the cast & riveted accessory rail. The integral machined rail was a big improvement on the full-size models (introduced for LE agency sales after commercial production had been stopped), but as far as I've ever heard, it wasn't introduced for the compact guns. I've seen a handful of new TSW duty guns that experienced broken rails (looks like a rather lightweight casting), and having the rail become part of the stainless frame is certainly an improvement.
I've always considered ordering a 3913TSW, if & when some became available as LE production surplus, but that's apparently not happening as much now that production was shifted to the Houlton plant. Also, I'd really rather have one made without the lightweight riveted rail.
Well, while we're wishing & hoping, I'd really like to see them make a 3916TSW, meaning an all-stainless version of the 3913. Yes, it would be heavier, but not quite so much as the mini-boat anchor 4516.

It would lend some increased controllability to an already outstanding compact single stack TDA 9mm, I think.
As if it would ever happen ...
Anyway, the TSW's are nice improvements upon the original standard 3rd gen guns. More robust & durable. Better inherent potential accuracy & controllability.
Improved, in most ways.
They really could've been called "4th gen" guns, if the company had wanted.
