I like the 4on4 for 5 minutes, then 3on3 for X minutes.....
Either play 3 on 3 for multiple periods until someone scores, or call it a tie after the OT period (of 20 minutes). As Silver said, a tie would be pretty rare. You could also extend 4on4 to 10 minutes and then go 3on3 for 10 minutes.
Of course, you could also just keep pulling players until it's goalie on goalie (tongue in cheek obviously, but paints a pretty funny picture).