I'm running LR4 on a pc. Control-Q does not close the program. (Sometimes it changes the selected image from color to b&w.) I've been closing the window by right-clicking the LR icon in the taskbar, and selecting 'close window'. Does this really close the program? (It also changes an image to b&w.)
File->Exit, or Clicking on the red X at the top right, should work.
Ctrl-Q should also work unless there is a Windows keyboard command that overrides it.
Try [ ALT+F4 ]