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.
Once you add friends through your Epic account (which has to be done via the Epic Launcher on PC or mobile), you can party up with them in-game from the lobby in the options menu, regardless of platform. Cross-platform matchmaking is then enabled when grouped with players on other platforms. It's important to know that if you're in a party with a PC player, you'll be dropped into a match against other PC players that are most likely using keyboard and mouse controls.
We have Fortnite: Battle Royale covered from all angles. Season 6 just started and we have all the Streetlight Spotlight dance locations marked down, all the cosmetic items that come with the battle pass, and the location of the secret battle star. For more on cross-play, be sure to read what Shawn Layden (SIE Worldwide Studios chairman) had to say about the future of cross-play.