I would unhesitatingly claim that Alt-SpaceBar (not Alt-F4) is my most frequent keystroke combination in Windows OS. I am not sure if this key combination is generally used by other users, but I love this combination, as it allows me to close, minimize, maximize or even restore window position by just using keyboard and therefore another reason of not using mouse. I can now keep my hands on the keyboard even if I require to perform some windows management activities.
Here is the Alt-SpaceBar keystroke combination list:
Alt-SpaceBar + c : Closing Window
Alt-SpaceBar + n : Minimizing Window
Alt-SpaceBar + x : Maximizing Window
Alt-SpaceBar + Enter : Restore Window
For example, to close a window, simply press Alt and SpaceBar, which will open a control popup menu, and then press c. These key combinations are more reachable to the users as it much closer compare to F4 which is at the top of the keyboard.
It might seem bit awkward and complex at initial stage but with regular use, I can assure you that you will love these key stroke combinations.