Daa 1.8 FE has no OSS, therefore cheaper 

Yes that works. The polarizing filters also have a rotary thread on the front where the filter plug-in system can be attached. 

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. 

there are 7 filter ring adapters, included 52mm/ 58mm/ 62mm/67mm/ 72mm/ 77mm/ 82mm, allows you use the nd filter on your kinds of size camera lens. 

Canon has at least 7 different 50mm lenses. You need to look on your lens to see what filter size it takes. It'll be written on the lens at the front, beside this symbol: Ø.