what do we do?

[Kidria]

If you're only up to Heavensward, you'll also need Stormblood on PS4.

If you're on the same FFXIV service account, sadly you won't be able bring up your character. If it's two separate accounts, then once you apply the Windows/Steam version to your account, you can bring up your characters from the PS4 version.

[Rongway]

If you're only up to Heavensward, you'll also need Stormblood on PS4.

This is incorrect. You only ever need to have (the base game) and (the most recent expansion) to be current. The separate expansion registrations only matter in the future when new expansions come out. Registering the latest expansion always includes the previous expansions.

Brightamethyst’s advice is accurate.

1. Decide who will continue to play on PS4.
2. For PS4, buy Shadowbringers only. The Shadowbringers license will apply Stormblood automatically.
3. For PC, buy either (starter edition and Shadowbringers separately) or (the complete edition), whichever is less expensive.
4. Register The complete edition to a new service account. If you put it on the same service account as the PS4 registration, you will not be able to log in at the same time.

This is the minimal setup to get you both playing at the same time. It comes with some caveats. You will each have to pick one platform and stick to it. The PS4 player cannot play on PC. The PC player will need to start a new character and will not be able to play that character on PS4.

Optional, but costly, additional setup steps:

• Buy a job boost and partial story skip for the person who will be moving to PC. You don’t have to do this, but one of you will be starting fresh on PC and this will cut down on catch-up time.
• Get a controller for PC. You’re probably used to game pad controls, so this will make the transition smoother.
• Buy an additional PS4 license for starter edition for the person who will be playing primarily on PC. This would allow them to log in on PS4 but only if they logged out in an ARR or HW area on PC, and while on PS4 they would only be able to do ARR and HW content.
• Buy an additional PS4 license for (starter edition and Shadowbringers separately) or (complete edition) for the person who will be playing primarily on PC. This will allow them to play on both PC and PS4 with no content restrictions.
• Buy an additional PC license for starter edition for the person who will be playing primarily on PS4. This would allow them to log in on PC but only if they logged out in an ARR or HW area on PS4, and while on PC they would only be able to do ARR and HW content.
• Buy an additional PC license for (starter edition and Shadowbringers separately) or (complete edition) for the person who will be playing primarily on PS4. This will allow them to play on both PC and PS4 with no content restrictions.

Again these bullets are optional. The first ones amount to convenience and the last ones are for flexibility. But as you can see, the flexibility of having two separate service accounts that can each log in to both platforms gets expensive quickly. However, the game is almost identical between PS4 and PC, and with a controller you’ll hardly be able to tell the difference, control wise, so picking a platform for each person to stick to should be fine.

[MicahZerrshia]

If it's two separate accounts, then once you apply the Windows/Steam version to your account.

Just going to throw this in, don't buy the game on steam for the PC. Then its linked to steam and you need to buy everything like future expansions via steam as well. It's best to just buy direct from the SE online store so you don't need to worry about any in between services. If you buy from SE direct you can buy expansions and what not from them as well in the future or buy from amazon or any retailer that just gives you the code to input, plus the game downloads direct from SE. Also if you ever lose access to your steam account for whatever reason, you won't lose access to xiv too.

[ItMe]

Just going to throw this in, don't buy the game on steam for the PC. [...] It's best to just buy direct from the SE online store

Seconded.
Steam has led to all sorts of issues with 14.
Game being unavailable at additional times because of problems on Steam's end.
Steam versions have sometimes not gotten big content patches on time (which can also lock you out of playing at all until they have the patch).
It's just another layer to go through if you have any account problems.
Etc.

Steam sounds nice, but avoid getting 14 on it.