If you need a glavanized option, that is only available in the 6,000 lb model as well. The 4,000 lb unit is by far the more common jack in the industry.
The mpb 40 has a smaller frame & fork width, drive motor, drive/steer wheel & transfer case, load wheels, lift cylinder, etc. - suitable for lifting 4000 lbs. The MPB60 shares many of the component as the MPB 80.
So the parts that are interchangeable would be the paint, some decals & a few nuts, bolts & washers.