Cross platform

[Brightamethyst]

but Frizze makes me think I have to buy all expansion on both platforms.

This is correct. You have to buy the expansion twice if you want to play on both platforms.

What Cait was talking about is the collectors editions versions of the game, which are more expensive and come with some fancy mounts, pets, and glamour gear. It's completely optional and has no effect on the game at large. Those you only need to buy once. You could buy the PC collector's edition and the PS4 regular edition and still get all the fancy extras.