Question: can fhd monitor display 4k?
Answer: A 1920x1080 monitor can only display that many pixels. A 4k video has enough information to display four times as much: 3840x2160, so four 1080p monitors in a 2x2.. So your 4k video gets resized to 1920x1080; it becomes four times as small, and that's what you're viewing.