Alt-F4 is a shortcut in World of Warcraft to make your mage cast a spell that vanishes the player who casts it. It was unused and a mistake that was put in by the devs, and they might patch it. It's pretty useful.
"How do you cast the vanishing spell?"
"you have to hold alt and press f4"
"thanks that saved my life"
alt f4 definition: a shortcut that casts a spell in world of warcrafts.