Sony and Psyonix have today confirmed Rocket League on PS4 is now fully cross-platform, as the game is now part of the PlayStation Cross-Play Beta Program. To enable cross-platform play, you’ll need to go to Options > Gameplay, and check the cross-platform play option.
This means that for the first time ever, PS4 owners can play the game together with PC, Xbox One and Nintendo Switch players.
The game doesn’t yet allow PS4 owners to create cross-platform parties yet but an update will be released to allow that eventually. However, you can create private matches with people on other platforms.