I play on both the PS4 and PC quite regularly. SE decided to not offer crosspurchase with HW. I'm not paying $80 for the same stuff twice. Instead I got HW on the PC and kept the PS4 version vanilla. There are no FAQs for this so I'd like to be the first. I just got off chat with SE support.

If you make a character with HW content on it:
1) an Au RA race
2) HW gear
3) your original(not MCN, DRK, or AST) job is 51+
4) using new jobs

Be aware that you do NOT need a HW service to log in on that platform.

However, if you log out in a HW area, you WILL need to log back in on a HW-enabled platform.

For example, in my case, I could log my level 60 Au Ra AST out in Mor Dona on my HW PC and then log in as a level 60 Au Ra AST in Mor Dona on PS4. I could not log that same character out in Ishgard on PC on then log in on PS4.

Helpful to know if your a multiplatformer looking to save a buck.