500,000 copies of Ryu ga Gotoku 4 (Yakuza 4) have been shipped. In just seven days the game outsold Yakuza 3 that, according to Weekly Famitsu sold a gand total of 499,436 copies before the end of December. Looks like the popularity of the series is on a solid climbing trend.
On top of this, the announcement mentions that the series (including all four games) has sold more than four million copies worldwide.
We still don't have rock solid sales data about Yakuza 3 in the west, but this should be enough proof, for Sega, that the series' potential is worth quite a lot more than the meager budget used for the launch of it's western editions. Given a proper promotion and a decent localization (without cuts), it's quite safe to assume that a solid title like Yakuza 4 could sell very well even in the west, finally awarding Kazuma Kiryu and his companions the wolrdwide popularity that they deserve.
Of course this means that Sega West should actually put an effort into it, instead of dooming themselves to failure with questionable marketing and non-existant promotion. Maybe they should give a good look to what their Japanese colleagues do, and start putting some of their marketing budget where true talent is, instead of wasting it all on promoting mediocre games like Alien vs Predator.