Just a thought, but if you genuinely need focal lengths above 300 mm actual, have you considered using a telescope?
Something like a Meade ETX80 has focal length 400 mm and f/5 focal ratio, for under 300 UK pounds. Or for real reach a Maksutov-Cassegrain like the ETX-90 or ETX-125 (which is what I have) with focal length 1900mm at f/15.
I've not actually shot through it with my GH4 but it worked just fine with a Canon 7D.
Sure, the prices go on up and up, and you might end up wanting an eye-watering Apochromatic for serious photographic use but telescopes often come with pretty stable, reasonably portable mounts designed to take the weight of the telescope plus a camera hanging off the back.
Cheers, Hywel