Check your lens thread size which is beside a "ø" (diameter) symbol. Order this filter of the same size. 

Daa 1.8 FE has no OSS, therefore cheaper 

No, use the M42 to Fujiadapter. 

Yes if you get the Z mount to M42 adapter . I have Helios 58mm M42 screw mount lens and it is fine with adapter . Bare in mind everything will be manual , no electrical contacts on adapter . 

If the gh4 is a micro 4/3rds system then it will work. I have quite a few Nikon and Sigma lenses including the 50mm afs and they all fit and work perfectly. You will have to set your aperture and focus manually but the camera will make all the necessary exposure adjustments itself. Don't forget on a 4/3rds system your 50mm will be 100mm based on a full frame. Make sure your camera is a micro 4/3rds and not a 4/3rds, I have both.