m4/3 is mirrorless, while 4/3 is not. The (mostly) older 4/3 cameras were designed as DSLRs, complete with mirror box and optical viewfinders. Because of the mirror, the lens mount on the 4/3 cameras is far forward from the sensor. On m4/3 cameras, the lens mount is very near the sensor. Thus, while m4/3 and 4/3 share the same sensor size, they do not share the same lenses.