3 or 4 screw or 5 screw was the number of screw! on the side plate and the front trigger guard. For a while the S&W had 5 screw, 4 sideplate and one front trigger guard. In 1956, the upper sideplate screw was delete, in 1962, the front trigger guard screw was delete. But with the introduction of the Classic series in 2007, the upper side plate screw was add, more as a cosmetic change than a technical one. For some collectors, this is something they look for.
Yves