Do I need to buy it again?

[ElazulHP]

Can you even combine a ps4 account with the steam version?

Also, I personally would never use the steam version, because now you always have to start/update the steam launcher before you can start the game.

You can go from PS4 to PC. I did that so can confirm it's possible. You can even switch between playing on PS4 or PC on the same account. You do have to buy the game and all the expansions again though yes. However, once you pick either a non-steam or steam version you're stuck there. You can't go from Steam to Non-Steam version which really annoys me. I would personally NOT recommend the Steam version as well.