#0 - Oct. 21, 2010, 4:40 a.m.
Q u o t e:
Everybody, regardless of what level their Account is at, plays using the exact same version of the game client.
So if you have logged on recently, then you are running on the 4.0.1 game client version, even if you are only at the BC level and cannot access any of the WotLK content.
In your case, you shouldn't need to install anything. Just log into your account on Battle.net and upgrade your Account using your WotLK license keys.
Then, next time you log in, you should have access to WotLK content.