Rather than repeat it all I'm going to risk upsetting folk here. To fix it you re-encode the video in the aspect ratio you want or buy the DVD, BluRay or digital media in the ratio you want.
Again, I risk upsetting folk when I write that downloaded video is often not ready for 4K play. It may have to be re-encoded or obtained ready to use.
When I stream video from my PC using PS3 media server to my Bravia x85c, the image by is tall and narrow. The default display also is set to "fit to screen" and is letterboxed, but really its just cutting off the top and bottom portions of the image. If I change it to "expand" then it just removes the letter boxes, but the image is still stretched and tall. Anyone had an issue with this and know how to fix it?