I'm late to reply to this.
You're looking at that wrong, don't look at how many units have sold total, look at the yearly hardware sales. PS4 is expected to decline YoY, that means there's no room for growth, for it to do better YoY after a decline would be rejuvenation which examples are Kinect for Xbox 360 and Pokémon for Gameboy. The PS4 has been on the market for four years already, its yearly HW sales have shown that the audience it appeals to are niche, where it looks like the majority of the demographic consists of Males aged in their mid teens to late 30s going by what software sells well on the system.
Of course there are exceptions such as Minecraft but that manages to sell well on any platform it is on. (Look at the Vita)
And with regards to MHW and DQXI, those were the only games to pull people from the mainstream to go and buy a PS4, the problem though as has been seen by the sales data for the weeks following is that MHW and DQXI is literally the equivalent of FIFA and COD for Europe, the game appeals to mainstream audiences but, those audiences are not game enthusiasts, hence, they are only buying a PS4 for those games and not much else, so the problem is that PS4 doesn't have much other games that are supposed to appeal to that mainstream audience in Japan.
The PS4 will need a price cut to pick up sales but the problem is still the lack of software that appeals to a mainstream/broad audience. I thought that Fortnite which finally released on PS4 in Japan in March this year would have done something, but the hardware sales suggest it had very little effect. Why I bring up Fortnite is that Battle Royale games are popular in Japan, but now Fortnite is available on iOS and it is doing really well in Japan, so it becomes obvious that the PS4 won't be able to pull people from the broad audience to buy a PS4 for Fortnite because it is available on mobile phones, and even before that there was a battle royale game on mobile called Knives Out by Netease, you'll find even that game is doing well on mobile, that just reinforces the point that battle royale games arrived too late to the PS4 in Japan because those games are already available on mobile.