Why? Most cameras made after 2014 are more than adequate for most non-pro hobbyists. A recent vintage Olympus or Panasonic M43 camera is still pretty competent, and assuming you mean 4/3 by "full four thirds" and not "micro four-thirds", you can always get a mount adapter to use it with an M43 camera. IIRC, the E-M series might work best with the old 4/3 lenses from a focusing perspective though.
I love the stills output (esp the colors) from Oly cameras, but for some reason I don't quite like the handling/ergonomics.
I don't know if it's because I had a Panasonic LX3 way back when, but I find my GM5 way more usable than Olympus E-PL6 from a button and menu perspective. YMMV of course, but my GM5 and GF2 feel much more solid than the Olympus bodies (EPL6 and EP2) I have.
I had a Minolta 7Hi originally - fixed lens, but sophisticated for the time - then Olympus E620 (FT), and then the EM-5 MkII (mFT). So my experience has consistently improved, but isn't very wide.
With FT and mFT cameras, the Olympus menus for a long time (and perhaps still) have an unfortunate default setting of the 'Super Control Panel' being disabled. Almost everyone enables this excellent feature, but that requires some deep diving into the standard menu system, and sometimes some conflicting settings need changing. But (!) once you've done that, I've found navigating the settings & controls to be blissful.