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This was released in Japan roughly 2 days ago. Who knows if this is getting an American or European release! There is a demo available to download and play on the Japanese PSN(meaning nothing is stopping you from getting it).
The biggest change in this game is that is centers around four protagonist: Kazuma and three new characters. Each have their own story that eventually leads to the four them joining together as a team. This is great because by 3, the game's attacks(except for the eternally awesome heat attacks) edge was beginning to dull. Now you got 3 new styles to play as and even more FUCKING BRUTAL heat attacks. Now for a shit load of trailers.
In-depth trailer
Story trailer... with (unofficial) subtitles!
TV commercial
Kazuma: The motherfucking man returns.
Akiyama: From homeless to finance worker, all thanks to Nishiki! A master of kicks!
Tanimura: Given up any desire to be a cop in order to find the man who killed his father. A grappler!
Saejima: Been in prison for 25 years for murdering 18 of his enemies at once. A powerhouse! Has a rather interesting "revelations" method.
A note on these trailers: I see Majima without eye-patch! A flashblack scene is assured. Also isn't this the first time we ever fight a girl? Another note: it seems in one of the sidequests you train someone for a tournament.
The demo has two modes to play: 1) Story mode, which is just like the Yakuza 3 demo, maybe longer. 2) 4-battle mode, where you play one battle as each of the characters. Graphics are mostly the same, but I think I notice an improvement in the number of decent textures. In Yakuza 3, it was really mixed. It seems the leveling system got a big overhaul. You pick individual abilities to level up, instead of pumping exp into categories.