First of all, it isn't even completely true, that the PS4 had the full backing of Japanese third-parties. They released their games on PS3, PS Vita and 3DS. Heck, Persona 5 was released one and half year ago. Even then Atlus did release it on PS3 parallel to PS3. And if you look at sales: while American sales were a bit stronger than japanese for this title on PS4 (470K vs. 440K), on PS3 the title sold only significant in Japan (60K in the US, 140K in japan). PS3 was after all these year a thing for japanese publishers. And that isn't true only for Persona, many japanese releases doubled on PS3 and PS4. Or they did go with Vita. Sometimes even without a PS4-release.
And the reason for that is, that the sales on PS4 were comparatively weak in Japan. And the reason for that is - that Sony initially showed pretty clear their focus was in the west. It was not alone the late release for the console in Japan, it was that Sony made first-party titles mostly for the west. This has changed over the last year though. Sony shows more initiative, one thing for instance is Ghost of Tsushima. This shows results. The 3rd-parties dropped PS3 and moving their Vita-games to PS4 and Switch. But Sony has lost a lot of trust of the japanese gamers at the beginning of the gen. They don't get it back that easy. If they keep on it though, the PS5 might show better sales in Japan.
Yes PS4 had to share many games with PS3 or Vita in Japan, no different from it sharing most western games with XBO/PC and in the first couple years PS3/360.
Fact remains that most major publishers in Japan have given PS4 strong support. And when did Sony 1st party ever prioritize Japan?
When the herd loses its way, the shepard must kill the bull that leads them astray.