From what we've seen this year with incredibly low sales and stocks in Japan and some stock issues in the US as well, it's pretty obvious that Sony is not interested in keeping the PS4 alive much longer. They've put all their efforts in making the PS5 as attractive as possible and focused on making as many units as possible for a quick transition.
Though this is contradicted by them supporting the PS4 by releasing Horizon 2 on it next year possibly as late as Holiday 2021.
But the stock situation does indeed seem problematic. Amazon UK has no new PS4's available.