Kroeber M4  The Kroeber M4 Köller was a 2-stroke air-cooled horizontally-opposed piston engine designed and built by Dr. Kroeber & Sohn G.m.b.H. in Germany in the late 1930s. The M4 proved relatively popular, for powering the ultra-light aircraft and motor-gliders in vogue during the 1920s and 1930s. "Data from:"AEHS : HOAE Kroeber  There are several M4 engines on display, but one of the best preserved and presented is at the Polish Aviation Museum in Krakow.