Np, there are no micro Four Thirds lenses of actual focal length beyond 300mm, probably for the same reason that there are no current Nikon or Sony lenses for 35mm format beyond 600mm: very limited demand. But not nonexistent demand! Some of us would like even more reach, and for now the only option is adaptor mounting a lens from some other system. (TCs do not help yet, because current MFT long lenses are too slow to work with TCs; the 300/4 will help there.)
As far as angular Field Of View [FOV], as often described by "equivalent focal length", I am usually happy to do things like convert my "16MP, 4/3" format, 600mm equivalent FOV" to a "9MP, 1" format, 800mm equivalent FOV" by cropping to 3/4 of frame height and width, which is like using a 9MP, 1" format sensor. Most of my long wildlife shots probably do not need even as much as 9MP (my technique and a strong preference for not carrying a tripod sets the resolution limit!)
To allow for cropping, I sometimes like to compare FOV and "equivalent focal length" at a given resolution, as roughly measured by pixel counts: overall, I would prefer to increase my reach by getting a next generation 20MP or 24MP MFT body that allows more cropping rather than by getting an even longer lens or fiddling with a TC.