I looked into this at one point. Seems the -4 might be the hardest to find. That's the model that S&W started drilling and tapping for scopes although mounts for dots are also out there. DX would be on my must have list if I were collecting -4's.
Not all 629-3 were drilled and taped. Mine is not, but the neighbors is. Both are designated as 629-3.
DR505
That is a nice one! It seems those are less common, I never see a 4" other than the mountain gun up for sale. Maybe people just hang on to them. Has the trigger been changed on that?