If there were to be a Hidden Region 5, then there would be a new bunch of bounties with it. They are not usually added when standard regions are released. I'm not sure why there was no R15 bounty when HR4 was released, since, if I remember, they were released at the same time. Hope this answers your question.
Based on past patterns, if there is a HR5 it will add rare hunts to R12 and R13. It will add bounties for R15 and the eventual R16.
Bounties are added with hidden regions.
HR4 was released before R15. Couldn't add a bounty for a region that did not yet exist
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Bounties are added with hidden regions.
HR4 was released before R15. Couldn't add a bounty for a region that did not yet exist
iPhone 6, iOS 9.2
I sit corrected. And your explanation was clearer than mine.