Do you currently own Destiny on PC, through Battle.net? Have you ever played the game on PC? If you do not own the game on PC right now, then you cannot cross-save to PC at this time. If you have the game on Battle.net, then you could activate cross-save between your PS4 and Battle.net accounts. Destiny is not active on Steam, as the move does not take effect until Oct 1st. Once the game is live on Steam, you can install the game, start it up to accept user agreements, and reach the character creation screen. Then you’ll be good to link up for cross-save, and activate your PS4 characters as main, so you can play on Steam with them.