The game tracks your "licenses" separately for each platform. If you bought the complete edition(collectors doesnt matter, as that only gives you extra items that you can claim) on PS4, youd have access to everything on that platform. If you then for example bought the base game on PC and registered it to the same account, then you would be able to log into your account and play with any of your characters - but only in the base ARR areas. If you had an alt logged out in the heavensward city you wouldnt be able to access them on PC. You would have to log in on PS4, move them to an appropriate zone, then you could play with them on PC. In practice, it means that you buy the game on the platform you use the most and wait for the expansion to go on sale to upgrade your side platform unless you need it right away.