I think PS4 does pretty well actually. Sure, it is no PS2, DS or even PSP. But it does pretty good. This is shown by the fact, that pretty much every japan-centric game is releasing on PS4, even as PSV-support is dwindling. That shows PS4 is a relevant device in Japan. Yes, it took a while to get there, but it is now. And sales are fine I think. The device is years old, it is probably behind it's peak, so these sales are very fine. That there is a new device that sells more currently shouldn't let us conclude PS4 does bad. On the contrary, while I see the hit 3DS and PSV took since the Switch released, the PS4-sales proofed to be stable against the influence of the competitor.
I do mean in current weeks. I think the current sales are bit concerning as well if the baseline does decline further. I think it is doing meh overall as well, not bad but not good for the health of the dedicated gaming market. Given it is some what inevitable to see decline in that space.