As i understood it, You only needed to purchase the base 'vanilla' ARR for both platforms.
When you purchase the Expansion, Your whole account is flagged for Expansion content.

I hope this is the case, it makes a lot of sense in my head.

If it works that way, a new player wanting to play on PC and PS4 after 3.0 would need to buy a copy of XIV:ARR for both platforms, but only needs to purchase Heavensward (for either) and apply it to their account to play Expansion content on both platforms.