This is a little curious. I have conflicting information on the weight and driver diameter. No. 3 had 33 inch drivers and was a little lighter. Maybe the Monson brass decided that if No. 4 was a little heavier and had slightly smaller drivers, she’d have a little more tractive effort. Maybe someone at Maine Narrow Gauge can confirm that No. 4 has indeed 30 inch drivers.
#3 is certainly lighter than #4. It's the main reason why 3 was the "traveling" engine when it was at the MNG. That and the fact that #4 was in the museum until about 2002.
Depending on who you ask, #4 is about 18-22 tons in weight once all the fuel is added in. #3 is probably 15 to 19, but I'm not entirely sure. They have a more or less accurate breakdown of the 4 Edaville engines in the Mark I video "Edaville Railroad: Massachusetts' Maine Attraction" that I can't remember exactly off the top of my head.
The drivers on #4 are indeed smaller than those on #3. The idea went back to Vulcan's designing of #3. If a shop was making a "custom" engine, they'd take all the data provided by the railroad (ruling grade, distance, speed, expected load, etc.) and build an locomotive to do the work. My understanding was that the Hinkley's could each pull a single carload of slate out of the mines at a time. Monson #3 was designed to do 2. When they went back to Vulcan, they asked for an engine that could do 3 carloads. Whether or not 4 regularly did 3 carloads of slate or not is not something that I know for certain, but I do know that she can handle a 7 (and maybe more) car passenger train even at reduced pressure.
Both No’s 3 and 4 had the unique Franklin clamshell type fire doors. I believe these were unique to the Monson.
If this was unique to the Monson originally, then Edaville liked them so much that they put them on 7 and 8. All of the Edaville engines have clamshell doors.