Summary: Rikimaru doesn't exist and isn't even mentioned at all in this game in any way shape or form. You absolutly can't have a Tenchu game without Rikimaru. That aside, if you wanna look at the game itself, it's more or less just a dumbed down version of Wrath of Heaven.
Summary: It plays just like Tenchu 3, only harder. No Rikimaru here but you get Rin, a young female ninja who is more like Tatsumaru. Story is not exciting but you don't have to deal with the undead. No 2-player mode. Using caltrops is a must to win boss battles! There are new items and abilities to gain.