Is the quality of video the same of 1080p and 480p in a div with dimensions 854px x 480px? Does the pixel shrink matters? I want to output a .mp4 video into a 480p div .
Yes and no, if the display your user is using is a normal density display, there is no visible difference. However, if it is a Retina display or a mobile/tablet with a high density display then the video is significantly clearer (no blurriness) when scaled down.
Going with a 1080p to achieve this is a bit of an overkill. 720p is plenty enough to achieve this clarity inside a 480p div.