I mean, they made 'me in 3" and 5" - I'd think a 4"er would sell very well.

Probably because S&W did not perceive a market for a 4". The 5" didn't sell very well and has been discontinued. The 3" sold far fewer then the 1 7/8" guns did/do. Just like any business, S&W builds what sells.

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Gunsmithing since 1961

In my opinion two things killed off the 5” 60: its odd barrel assembly and inappropriate marketing. All the salesmen I saw show it or talk about it had no idea why anyone would want one. The 5” 60 should have been marketed as a target revolver for short fingered people---buy it as a 38 for your wife or teenager to use. Whether the 5” 60’s two piece barrel assembly functioned better or worse wasn’t an issue, but most revolver buyers like traditional construction.

The 3” 60s are sold for concealed holster carry for which a 3” barrel is about perfect.

I don’t remember any 4” J frame .38 specials or .357s. As best as I can recall S&W’s five shot 4” .38s died out with the .38 S&W Regulation Police. It could be S&W’s marketing department remembers how long it took to sell the final inventory of those while 4" K frame 38s were selling by the millions. There might be a market for J frame .38/.357s with barrels slightly longer than 4” in Canada where laws restrict barrels 4” and under. That’s the most likely motive for S&W to make a batch. Ruger is making 4.2” .357 SP101s and you can bet S&W’s marketing department is keeping an eye on their sales.

Personally, I'd make a bet that a 4" J-frame Model 60 would outsell the K-frame 4" Model 67.