Not many. I think they were "listed" during the 1970's, maybe longer. Some may have been made up as special orders over the years. Certainly, some older (pre-27's) were ordered as four-inch guns.

The late gun scribe Col. Charles Askins had a four incher, I think with a humpback hammer.

If you find a good four-incher, hang onto it. I've never understood why the four-inch barrel wasn't a longtime standard choice in the M-27. They were making four-inch barrels for the M-28, and it would have been a better choice than the 3.5-inch barrel.

I think most of the four-inch guns were made after S&W standardized N-frame barrels at four, six, and 8 and 3/8th's inches. I believe some may have made as a short run in the 1990's, but haven't time to look it up. Someone else will post that, I'm sure. I know they made up some five-inch guns then, but those I saw didn't look as good as I'd hoped. I had one that shot so poorly that I soon sold it.
All may not have been that bad.

T-Star