Alt + F4 switches to TTY4

I had this recently on Ubuntu GNOME and on Unity. The answer is this:

sudo kbd_mode -s

Run that in a terminal and then the Alt+F4 keyboard combination returns to normal.

From kbd_mode man page (from kbd project and package):

kbd_mode - report or set the keyboard mode

Without argument, kbd_mode prints the current keyboard mode (RAW, MEDIUMRAW or XLATE).
With argument, it sets the keyboard mode as indicated:

-s: scancode mode (RAW),