If you look at hardware sales dating all the way back to the NES days....all the way up to current gen... Japan always accounts for aproximately 20 Million hardware sales.

http://www.vgchartz.com/analysis/platform_totals/

NES= 19.3 Million sales

SNES+Genesis = 20 Million sales

PlayStation+N64 = 25 Million sales

PlayStation 2+GameCube = 27 Million sales

PlayStation 3+Wii = 22 Million sales

Now Wii U is not even at 1 million sales in Japan. PS4 is set to come out with a ton of momentum. Final Fantasy XV and Kingdom Hearts III have been announced and will be huge in Japan. Metal Gear Solid V will be big in Japan and will come out soon after launch. Sony's JapanStudio is making a comeback (Soul Sacrifice,Knack,Puppeteer,Rain,Gravity Rush).

Sony is in prime position to take back Japan. It looks like there's a good 20 million sales left for the taking in Japan. 

PS3 has sold 9+ million in Japan, and we all know it was extremely expensive in Japan and was overwhelmed by the original Wii when it came out. This time around Wii U has failed to take off and Sony isnt facing much competition in Japan. Plus the console will be much cheaper from day 1.

I think PS4 can sell atleast 15 million in Japan.... maybe even 18-19 million. 

       If  I said 9.78 million for Wii U in Japan, I will say 8.06 million for PS4.

It doesnt look like Wii U is going to sell even close to that number.... so it leaves a lot more potential PS4 sales. 

*cough* Animal Crossing * cough* Monster Hunter* cough*  Dragon Quest * cough*

Monster hunter is a handheld game. It does nothing for home consoles.  

Dragon Quest X did nothing for Wii and WiiU

OP:  10m at max for PS4, Japanese market is dying fast.

          Monster Hunter Tri sold 2 million. 0.37 million for a remake ( not including digital) is pretty good. Heck, Capcom said it was a smash hit so it may have even reached 1 million. The reason Dragon Quest X had meh sales is the same reason Final Fantasy XI had meh sales.. It was a MMO. Dragon Quest is known for being a single player experience. The game  had meh sales and rightfully so.

^this. JP console market will probably RIP this gen.

Well it's got Final Fantasy, though I'm not sure if the brand declined after FFXIII.

DQ, MH, Pokemon and all the big franchises are on 3DS. PS4 will likely be more of Western console than a Japanese one. I say WiiU+PS4 don't reach 15 mil combined. Japan's lifestyle is more suited to handhelds. Consoles are a dying breed.

Ps4 > 10mil sounds about right.

Rubbish. The most recent gen still sold over 20+ million hardware units in Japan. 

No change from the early NES and SNES days.