Need clarification on PS4 needing physical disc in system during Heavensward gameplay

[Mjollnir]

Related question, if I'm currently playing from a physical copy on my PS$ and go for the digital edition of the expansion, will I still need the disc in the system to play after release?

You shouldn't need the physical copy in your PS4 at the moment.

The only thing you might need your ARR disc for is if you ever need to reinstall, but it's quite likely you won't even need it then and can just download the whole shebang through the launcher.