Telephoto Lens Suggestion

Looking for a recommendation for a 300mm or 400mm lens to use with my GH4. Manual focus is ok and lens stabilization is not needed. Due to limited budget I can probably afford only an older lens. I borrowed a likely choice, the Panasonic 100-300 zoom, but at 300mm the image was pretty soft. I heard that the Canon 300mm F/4 L FD was supposed to be good. My budget is under $500.

$500 won't get you too far in that category. DO you mean 300-400mm FF equivalent?

Unless you are considering a canon FD "L" lens as a vintage cheaper alternative you aren't going very far,but that would be beyond your budget as well.
Since you are looking for such extreme telephoto lenses in the ballpark of 300-400mm, my suggestion is to find a used canon or nikon mount sigma 150-500 or 50-500 to accompany with a speed booster,which is still expensive,but has the added benefits that you can add more and better lenses that those offered for m4/3.

Thanks, I will look into that.