I downloaded Client 4 yesterday, and it worked well.  This morning, I could not open the client from the task tray icon.  I closed the program and tried re-opening it from the desktop icon.  I got the error "Curse Client cannot open its main window" and it gave the dimensions of the window (500x860).

I'm running Curse on Vista(x64).

Have you tried it later in the day? There was two updates today Tuesday for the client.