another idea... use the '2 minutes or less' rule and mail a $4 check to your friend. Include a note - "Hey pal - here's the four bucks I owe you from blah-blah. Didn't want to lose track of returning it to you, not sure when we'll see each other next."
That will finally get it off your mind altogether, it's worth the postage stamp to get this one off the radar screen.
I like this idea best of all. Especially for such a small amount of money: delaying isn't worth the system overhead.