You can not log into 1 account more than once.

You can play a PC account on PS4 if you register a PS4 game key to your account. And vice versa.

It will allow you to play the game on either PC or PS4 with that account, but you should be aware that expansions are exclusive too.
Buying Heavensward for PC does not unlock Heavensward for PS4. If you want to play on both PC and PS4, you need to buy Heavensward twice.

You can use multiple accounts on 1 PS4, you simply log in with with another user.