I just purchased a 625-3 that has a 4 inch barrel.
Sho nuff the later stainless 625 has come with 4" tubes, but back to the 25-2, which is an entirely different breed of cat . . .
Back "in the day" S&W really missed a great market with their .45ACP target revolver. Only in the last few years did they offer a 6" barrel option to the normal 25-2 with the 6 1/2" set up.
Lots of folks back then, understanding this model's incredible potential as a carry gun, had 'em chopped to 4," 3 1/2," 3 1/4," and in the case of the Jovino customs, sub 3" versions. Some smiths did great work . . . and some mediocre of course.
I stumbled across this one years ago . . . made in Savannah I was told back "in the day" by a fine wheelsmith (now unknown) who has long passed away.
Mine retains its N frame square butt, though others were converted to N frame OR K frame round butt versions. After I got it I radically narrowed and rounded the original wide, serrated "target" hammer for double action work in handgun matches and put that "dreadful" MIM cylinder release latch on the side (since I'm left handed an this latch works much better in fast, double action matches).
Mine wasn't ever told it is no longer a 1980 6" target revolver . . . for it still shoots incredibly tight groups indeed . . . and the trigger job is jaw dropping. It is the very last gun I'd ever part with, I love it so much. Here's my 3 1/4" dream gun, and I'm sure a 4" version would also be very sweet . . . though not quite as "concealable:"
I think it makes a great complementary "piece" for my 27-2 Model 27 . . . in original 3 1/2" barrel version of course . . .
Good luck on a successful conversion. The odds of finding an original 4" version are slim to none, as others have detailed.
Tom