In a recent interview with IGN Rocket League developer Psyonix confirmed that it had solved the technical issues surrounding cross-play between between PlayStation 4 and Xbox One users. And that adding the feature to the game would be as simple as flicking a switch. So what's the hold-up? Well, they're still waiting on the go ahead from Sony.
Currently Rocket League features cross-play between the PS4 and PC as wells as Xbox One and PC. So the missing coupling is pretty clear. In the interview the developer pointed out that with a company as large as Sony these sort of things can take time. Naturally, but still. C'mon Sony! Make it happen.
And after the new broke, Gwent and The Witcher developer CD Projekt RED confirmed that cross-play between consoles would also be added to Gwent once Rocket League was given the go ahead.