What do you mean by activating my HK account as primary to play from my US account?
Whenever you create a new user on PS4 and signing in a PSN, there will be a bunch of introductory screen and one of them will ask if you want to activate this console as "Primary", if you cancel it, it will be activated as secondary.
The difference between Primary and Secondary is:
Secondary is an account that is temporarily registered on a PS4, for example you went to a friend's place and you want to play your own digital game/progress for that time being, so you activate it as secondary. After you are leaving, you can just delete it.
Primary is meant to be among family members, where your brothers or sisters have their own accounts and everybody is sharing the same PS4. By activating primary, all of the contents are shared across accounts as long as it's on the same PS4.
You can only have one active Primary or Secondary at the same time, you can deactivate it anytime too.
I dont think you need to do that. I've downloaded games (albeit free ones) from the JPSN and was able to play them with my standard american account
That means you've activated it as Primary. It's just one of the starting screen when you are creating a new user that some people skipped it, which will cause the PS4 to assume it as secondary. I always see the same complaints around here that they couldn't play any contents from other account, so just an extra note for everybody
