The differences between the 4-digit S&W Semi-autos is in the model numbers (well, sort of).
First off, (and this only pertains to semi-autos - and I don't believe any of the plastic guns), there are 2-digit models, 3-digit models, and 4-digit models. The older models (with 2-digits or 3-digits) cannot handle +P in almost any case (if someone knows of an exception, by all means, feel free to correct me - I'm still lost on a lot of the S&W alphabet soup).
639 can handle +p 2nd Gen.
The 4-digit model numbered guns are made up of 1st gen, 2nd gen, and 3rd gen - so don't let anyone tell you that just because its got 4 digits, its definitely a 3rd gen gun (I used to think that).
39/59 series 1ST Gen, X39/X59/X69 2ND Gen, Then we come to 3RD Gen, all rules are thrown out the window, cause this is where S&W started to combine the 3 digit/4 digit, most of this was done with the 45ACP 457 serie's, but there are others!!!!!!
XXxx
In the 4-digit model-number system, the first two digits indicate caliber and "basic model system" which I think translates into single/double-stack from all the models I've seen:
The "39xx" series is a single stack 9mm compact. The 59XX series is a full sized 9mm as are the 69XX series.
69XX is compact model, same size as the 3913, but with a 12rd magazine.
xxXx
Third Digit is "Type of Model" which indicates size and action-type (I think). "0" is standard model, 1 is compact, "2" is standard with decocking lever, "3" is compact with decocking lever. "4" is standard with DOA action, "5" is compact with DAO.
The 3913 is a lightweight compact alloy frame. The 5906 and 6906 are also on lightweight alloy frames, but are full-sized.
5906 is all steel, and 6906 is alloy frame.
xxxX
The last digit in the designation means the frame material type. 3 is aluminum frame / stainless slide, 4 is aluminum frame / carbon slide. 5 is carbon frame / carbon slide and 6 is stainless frame / stainless slide. 7 is stainless frame with carbon slide.
Letters after the 4 numbers are just accessories added onto the weapon ie: "TSW" means Tactical Smith & Wesson - which basically means it has a rail. LS means "Lady Smith" and the weapon usually has some performance center work as std for the model and other design cosmetics.
The problem is that Smith didn't stick with their own system as there are lots of exceptions to the above set of "rules".
Damned if that isn't the truth!!!!
Smith actually released a "secret decoder wheel" at one time (here's a link about it):
Its not even correct, I've got one!!!
EZB, Don't forget all the different -dash series to add to the confusion!!!!!!!