I purchased a 3" Model 29-4 in 2006 from the dealer that sold it to the original owner. At that time I was told that it was a very limited edition made only in 1989. When I got it home, I started digging in my Smith & Wesson bible and here's what I found. The 3" Model 29-4 was indeed made only in 1989 and was limited to just 2532.
My 6-1/2" Model 29-2 is a lot more pleasant to shoot with full-house magnum loads than the 3" version. It's very difficult to control the 3" in recoil. It just wants to jump right out of my hands. I haven't started reloading my own yet, but will before much longer. I will become friends with the 3" yet!
Willie