In the Kodi System settings, Video, Playback, "Display 4:3 videos as" , set it to Stretch 16:9. If you don't see that setting, click on "Settings Level" until you do. Now all video should fill the screen, except programming with black bars included.
If you are looking at something that should be displayed as 4:3, click on the movie reel icon when watching a video and select the right format from the menu you see there.