Jim 44-40, Welcome to the Forum. If you are referring to Smith & Wessons, the label of "4th change" has been applied to several models of older revolvers for the reason that some engineering change was made in the way the gun was manufactured. This happens often when a gun has a long history of being manufactured and new designs or techniques are developed over time and then applied to the manufacturing processes. Changes usually are in numerical order - 1st change, then 2nd change, etc, up to whatever is the latest change, before the gun is dropped from production. The M&P Model of 1905 is the model you see most often referred to as a Model 1905, 4th change. In that case, the change was mostly minor internal changes, as opposed to the Model 1905, 3rd change. When S&W changed over to using the Model number to designate a gun, in 1957, changes thereafter are indicated by a dash number after the Model number, such as Model 10 - 3, etc. Ed.