Alt F4 is a function (specifically in Windows) that seems to be a running prank on newbie computer users in general. Although its usage is prevalent in just about any location, (the workplace, home, even friend's houses), its popularity seems to stay pretty constant on IRC. The gag usually goes something along the lines of:

NewIRC'er: Hey. How do I get into #movies4free? It says 'this channel is restricted' when I try to join and I can't get in.
FunnyMan: Oooh. You want into the movies room. Hm. You have to know the secret code, my friend.
NewIRC'er: PleezPleezPLEEZ tell me. Pleez.
FunnyMan: .. aah, ok. you see that button "Alt" at the bottom of your keyboard? The one near the spacebar?"
NewIRC'er: (excited): Yeah!
FunnyMan: K. Hold that down. Then, press the F4 key. That'll unlock the hidden menu on mIRC, and then you can get in there.
NewIRC'er: Hey, thanks man. I really appreciate you helping me. Lemme try that real quick.

*** NewIRC'er quits (Client closed connection)

FunnyMan: hehe. Gets em every time. *grin*

In short, Alt-F4 closes the active program/window. Therefore, say you were on IRC, and it was your active (in-use) window. Press Alt-F4, and... wham: you've closed the program. People fall for it more often than you'd think. =\