https://www.gamespot.com/articles/how-to-activate-fortnite-cross-play-between-ps4-an/1100-6462112/

Sony recently opened the PlayStation 4 up to cross-platform play "for select third-party content" which is big in particular for players of Fortnite--the first game to officially enable the functionality between Xbox One and PS4 users. This also means that account progress and cosmetic items are now platform agnostic, though these functions are currently in beta form. But there are a few things you'd need to do to activate matchmaking with players of all platforms.

First thing to note is that cross-platform matchmaking is only available to those who are in a party with people through Epic Friends. You'll need to group up with others via an Epic account; this is the ticket to connect with players outside the platform you play on. In order for console users to access that, you'll have to make sure your Epic account is linked to your console account (PSN, Xbox Live, or Switch Online) which can be done on Epic's official website.