The 4020 is the old series prior to Eastman arriving on the scene. It has the traditional hand control for speed and direction and was updated by Eastman to provide a tighter turning radius. It is available with power steering and has a PS added to the model number to signify that.
The 3100 and 4100 series were only made in 1992, 1993 and 1994 and they are the All Hydraulic models that did not belt drive any attachments.
The 4200 series also has the tighter steering and is available with power steering but you can order it with a foot pedal that controls the speed of travel only. Direction is still controlled by a hand lever.
That's my understanding but bhildret should step in here and correct any errors I may have made.