Only you'll be able to start the games you've downloaded, and you can only be logged into two consoles at any one time: your primary PS4 and one other secondary one (like a friend's). If you want to play on another PS4 that isn't your primary console, you can download any game you've bought on the PS Store as long as you sign into PSN. Additionally, if you have PS Plus and register a PS4 as your primary console, then every account that plays on that system will be able to make use of certain benefits like online multiplayer even without their own PS Plus subscription. Registering your PS4 as your primary system will allow other users who play on the same console with different accounts access to any games you've downloaded without you having to log in.
