It's just a personal preference on his part. And the fact that you can get an M2 for less $$$. The main differences are: the rapid loading system and angled rewind knob on the M4.
The M2 uses the somewhat less convenient(IMO) rewind knob that has no lever and you need to pull the film spool out to load film into the camera. Oh, and the frame counter has to be manually reset. Just turn the dial!
They share the framelines unless the camera has been modified. 35/50/90/135.
The M cameras that have the 28mm lines also have the .58X viewfinder which means a shorter base RF = less accurate focusing. Higher magnification=more accurate focus......But, with the wideangle lenses you can use hyperfocal focus & not worry about it.
Just as an aside, I wouldn't worry about size & weight of fast lenses on the rangefinders The size & weight difference you can judge just from a couple of spec sheets. The Leicas lenses are going to be smaller for a comparative f stop but the materials in the Leica lenses will be heavier. Lotsa brass, no plastic/motors.
You can use the screwmount lenses by means of a relatively inexpensive adapter