1080p is a 1920x1080 rectangle ( assuming not a "wide screen/anamorphic film). a 4k TV is a 3840 x 2160 rectangle. the small rectangle when shown a “original” size is much smaller than the TV. If your movie was 1920x1080 and your tv was 1920x1080 then you would not see any difference when set to original because they are the same size. if your movie was 3840 x 2160 and the tv was 1920x1080 the it would appear zoomed in.
Letter box increases (or shrinks) the size until it fits the screen with nothing cut off and no distortion. Left/Right in the case of wide screen touching the sides, which is most common, and Top/Bottom in case of things like 4:3 or vertical phone footage and nothing gets cut off. This is the default on all clients so the content fits the screen and you can see all of the image without distortion. if your movie and tvs resolution matched you won’t see any change
Zoom is basically the opposite of letterbox setting in that it zooms to fill rather than fit . On a wide screen movie this will cause the left and right sides to get cut off and and the top bottom to match up with TV. this also has no distortion beside the sides getting cut off. many folks use this when they don’t like the top bottom “black letter box bars” as most of the important image is in the middle of screen anyway. (folks who appreciate cinematography generally don’t do this.)
Stretch fill the tv screen with the image, will not cut off, but will distort. if you have a wide screen image that normally would have the black bars at top bottom it will stretch the height till it fills up the screen. this results in things being skinny. if it was a 4:3 image then it will stretch the width and things look fat.
also some video files have the black bars as part of the image. most dvd/bluray ripping apps crop them out by default for sake of getting the file size smaller and have the tru resolution show in specs. There are also some movies which have part of the movie filmed in IMAX and the other in regular. Examples the Dark Knight opening bank robbery was filmed in IMAX and fills screen it goes into letter box in following scenes. The movie Tron Legacy is another when in the “real world” it is a letterbox widescreen and when they are in the program world of “the grid” it fills the screen.
