I know that cross-save lets you move your account to a different system you were previously playing on, but can you go back and forth? For example, can I play on pc and then ps4 then back to pc?
You tie a single account to cross play. This will become the master account. Choose the platform on which you have made the most progress. You can now play That account on any platform except stadia. The down side is that this isn't like Microsoft cross play. You do not get the game itself on multiple platforms by buying it once. You need to buy content on every platform you want to play on. Potentially this is a super expensive thing.