You don't even need to use Alt+F4, you can just hit Esc and move the mouse to the top right and hit the small/big window button (not literally, though, as you will break your computer).

There is also another way to get stuck in the fullscreen, and that would be if you hit F11. In that case just close Minecraft and restart it. It should then be back to normal.

F11 for Fullscreen

By default, F11 toggles fullscreen mode.

If that doesn't help, pause the game (by pressing Esc) and go into Options… → Controls… and check your keybinding for toggling fullscreen mode (under the "Miscellaneous" section).

Alternatively, you can go into Options… → Video Settings… and click "Fullscreen: ON" to turn it off manually.

Go to %appdata%, click on the .mineraft folder, and in the search bar search for options.txt.
Upon clicking options.txt, you can find 'Fullscreen:true'. Toggle the state from 'true' to 'false', and relaunch Minecraft. You will no longer be in fullscreen mode.

If your not okay with Mincraft covering your screen fully, you can press F11 this works in most of the times, but if it does not work you can try

1. Close MINECRAFT (including mincraft launcher) start it again.

2)Open minecraft manually press "Esc" go to options--controls--see the key for your toggle fullscreen and use it

3. go to minecraft press "Esc" go to options, go to video settings, there you see the option to exit fullscreen..you can press that option THANK YOU