What is the benefit of APS-C over 4/3

This is true if you hold the f-number constant while adjusting the focal length to have the same field of view on the smaller sensor, but holding the f-number constant does not produce an equivalent lens.  The f-number has to be scaled with the crop factor.

It's the available lenses for a format that determine the depth of field you can achieve, and this is where FF has the advantage: there are many FF lenses that have no small-format equivalents.  When there is an equivalent in absolute specs, often the small-format lens has more imperfections.

I agree that APS-C is a good compromise between size & weight and DoF control, given the currently available lenses.  The large-aperture lenses that Fuji and Zeiss have been cooking up for X-mount (and even the Samyang f/2.8 fisheye) have vastly improved the value of APS-C in my eyes.