MacOS – How to get back the missing monitor option in Mountain Lion’s menubar

You could run 2 DVI/VGA montiors via the thunderbolt ports with adapters, and 1 via HDMI. I believe the 4th would need to be connected with a USB-to-vdeo adapter.

Check out this question for some more information: How many external displays can I power with the 15" retina MacBook Pro?

Do you mean you want to leave the secondary monitor plugged into the computer, but not use it?

You can go to System Preferences > Displays > Arrangement and check the Mirror Displays checkbox. Make sure your primary display is set to the laptop's built-in display. Then just shut the secondary monitor off (so you don't see the mirrored display).

Now all your windows will be on the primary monitor and you won't be able to drag any to (or open any on) the secondary monitor, but it will still be plugged into the computer.