The PlayStation 4 has sold amazingly well since launch, even without its home nation of Japan not included until mid-February. Sony's console is outselling the Xbox One handily around the world and the Wii U is struggling to move units, so adding Japan to PS4's markets was expected to boost sales even further.
However, the PlayStation 4 hasn't done as well as speculated in Japan, and the Wii U's launch weeks outpaced what the PS4 is seeing now. Through the first three weeks of each consoles' launch, Wii U sold 557,901 units, while the PS4 sold 410,083.
This isn't a huge discrepancy, but it is worthwhile to note against the narrative of the PS4's relative success and Wii U's struggles. These differences could be attributed to a few different things, though.
For one, the PS4 has been limited by supply constraints in most of the world, with availability scare in western markets. It's unclear if the same constraints are plaguing Japan, but if supply is limited even a little, it could result in the roughly 150,000 unit difference between the consoles.
Another factor is that the gaming hardware market in Japan is down in general--the 3DS is trailing what the DS did, and last-gen console sales didn't hit their former heights. Many people in Japan are focusing on gaming on smartphones, which are in general more powerful and advanced than those in the west.
I would also cite a lack of quality software for the PS4 as a reason it's not selling that well, but I don't think that's useful when comparing it to the Wii U, which didn't exactly light up the market with great games.
Better games will spur the PS4 on to more sales, no doubt, but when comparing the two at launch they both suffered from a lack of good games. The Wii U edged the PS4 at launch, but only slightly, and the console is very healthy overall. As supply grows and better games come out in the future, (perhaps some aimed at that audience) I would expect the sales to pick up.
Source: IGN