The image is regrettably tiny, but detailed enough for us to see what's going on -- and what's going on is that Rikimaru is cutting the head off of whoever picks the game up off the shelf. We've enlarged the image a bit. It's not as beautiful as the Japanese box, but it does have a certain "video game ninja" quality about it.
We're not sure the name change was the way to go; while Shadow Assassins is both more descriptive and exciting than 4, if we were Ubisoft, we'd want to emphasize however we could that this game was somehow different from all the other non-numbered Tenchu games not made by Acquire.
If you're in a Tenchu mood, be sure to browse the official website. And if you're Australian and in a Tenchu mood, good news! The OFLC has rated the game for release.