It's only the first part of the entire story. There could be 2 more parts after this or more. They haven't said on how many parts will be exactly with the remake saga and yes I believe the entire saga will be playable on the ps4 or at least supported for the PS4.
“The next-generation consoles will have backward compatibility, so we plan for the time being to make our new titles available for both current and next-generation consoles,”
Though I am sure they will make an upgraded ps5 version of the entire saga eventually. Despite all that FF7 Remake Part I is an excellent jrpg game, while story is not complete it has satisfying enough conclusion imo with an epic final battle already.