Daa 1.8 FE has no OSS, therefore cheaper 

No, there are 6 filters. 

No, he filters 6. 

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. 

it depends on if your polarized filter has threads on both sides if so yes. if not no. This ND filter does not have threads on the outside, therefore you can't stack another filter on top of it.