It'll say in your license which versions are covered. There was a window of time where a v3 license would also cover v4, so you may have been lucky, but you'd have to look at your own copy to find out.
- Does my license cover Grapefruit Juce? - hopefully yes
Yes, that's just our trendy branding name for V4, so it's the same thing.
- Does my license cover all Grapefruit Juce platforms? - I hope yes, but it's really not clear to me
Good question.. Since the old licenses didn't mention platforms at all, I think it's fair to say that it does cover them all.
- Does my license include access to the Projucer? - I also hope yes, but I'd understand if not.
I'm glad you're understanding, because it doesn't! We're using that as a bit of leverage to hopefully get you to upgrade ;)
- Do I need to have a Roli account to use Grapefruit Juce/Projucer and how should I get one - or do I already have one?
If you mean "do I need a MyROLI account, or can I use my JUCE forum account?" then the answer is that our web guys haven't unified the user databases yet, so annoyingly, you will need to open a MyROLI account. Sign up is here: https://accounts.roli.com/en/users/sign_up
Or, if you mean "can I use the Projucer without any kind of account", the answer is "sort of..", because if you cancel its login screen then you can use the non-compiler features of it, which is basically the same as the Introjucer. But you will need to register a proper account if you want to use it properly.