As far as I know, the “four thirds” mount was an open, with the same sensor size as its successor, the “micro four thirds” mount. The former was a DSLR system, the latter is a mirrorless system. I don’t think there are any commercially available interchangeable lens cameras with that aspect ratio, that don’t use either of the two standard mounts - they are both from the Micro Four Thirds Standard Group (link).

Canon latest mirrorless bodies are awesome and also good value (but stay away from the R100). As others have said, good native RF lenses are expensive. They have also cheaper options, but what they miss is the good value middle ground usually represented by Sigma and Tamron. One big upside is the EF to RF adapter, that works very well and lets you use the lens you already have before investing. Remember that a lens remains with you decades, contrary to a body, and paying a bit more could really be a good choice in the long run. Just my view, a bit in contrast with many steering you away from Canon. Quality wise, there’s no difference between brands: most importantly a camera should disappear in your hands and be really ergonomic - go in a shop and handle them all!