Do I need to buy it again?

[DumdogsWorld]

If you own the game for both PC and PS4, you can log in on both platforms. You gotta buy the game twice, though.

Your character data will work out of the box, but hotbars and other locally saved data need to be transferred.

[Avidria]

I would personally NOT recommend the Steam version as well.

Seconding this^

If you get the base game through Steam, you need to buy every expansion through Steam and also have Steam running in order to launch it and while you play. Nothing but an added layer of annoyance... the only upside being that I can use any Steam gift cards I get on mog station things I guess.