If the PS3 with low price and a big game catalogus cannot do, why would the PS4 do better?

No, but it can slow the decline.

Doubt it. Traditional gaming is declining in Japan, not just console gaming. We thought Japan was a safe haven but it is actually worse off than the West where decline is primarily due to long console cycle and Wii's install base moving away. In Japan there is a shift from console to handheld to mobile/casuals, the devs couldnt keep up with the HD transition.

What was that report that Japan leads the world now in spending on mobile apps with the no.1 app categorey being games?

You can see why pubs like Square-Enix and Capcom are jumping ship.

No ... sadly I don't think the PS4 will do much. The PS4 seems more Western-centric than the PS3 was even. There's not many Japanese devs left either that I think can make huge budget PS4 games.

They will be sticking to the 3DS or PS3 for a while in a lot of cases I think.

Even though I wouldn't have really thought it a year ago, I think some type of Nintendo phone, maybe even primarily for the Japanese market is coming. Perhaps a Playstation Phone (a real one) too. 

Nah. Japan has passed its peak for home consoles. PS4 will sell well, but it + Wii U won't match PS3+Wii, which in turn has failed to match PS2+GC.

Japan's gaming scene is becoming more and more irrelevant.

With FF15, KH3 and a Monster Hunter game it could do well in JP.

While I don't think this gen will be as grim as some make it out to be, I also don't believe we will see an increase in how many home consoles are sold there. It will most likely see another decrease of ~2M, so ~22M total when all is said and done. PS4 will probably see ~12M of that. The Xbox One will get ~1.5M. Which leaves ~8.5M for Wii U.

I don't see it changing anything, especially since we all know Japanese developers are still gonna be making games for the PS3 for a while, not the PS4.

  Simply no. I actually see the Wii U doing much better than the PS4 in Japan.