Even the PS3 sold 10m in Japan, there is no reason why PS4 can't reach 15m. Considering:
1. Wii U is no Wii this time around, stealing Sony's console hardware sales. PS4 is the only player in town, so Japan has no other choice. Full Japanese support.
2. The Western games adoption by Japanese is getting bigger and warmer, see how well GTA, Call of Duty, Destiny, etc are selling in Japan. Xbone is no X360 this time around, so PS4 is getting full western support unlike the PS3. PS4 being huge in Western countries will be translated to the Japanese market in a year or two. This gen will be named as the Japanese gamers warming up to Western games gen.
3. The tag team of Vita and PS4 are working far better than Vita and PS3. Games designed for the Vita will be ported to PS4 and vice versa. Easy as a click of a button.
4. You don't know what Sony and others have in store for PS4 yet.
5. The lifecycle of PS4 will be longer than PS3.
For all these reasons, seeing how PS3 can sell 10m in Japan. 15m for PS4 is being conservative.