No need to download plex app on ps4, just download the app on your pc. goto plex.tv website. and then after installing the app it will open the browser homepage of plex application on your pc, make a free account, select the folder in which you have movies, then goto ps4 browser, goto plex website, sign in with your account and Viola. you can stream movies on your ps4 from your pc, same 720P or 1080p movie quality and great surround sound.
please goto Ytpak.com (Pakistans Youtube solution) and search playing movies on plex ps4, and follow the second search link and follow the video if you face any problems.
the service is totally free. with a great browser interface.