I always tell when I'm going to Q4....by either saying (usually around 5 or 6 minutes past the :00 or :30) "Need just one more and we can have a Q4" or "Just one more in and I'll Q4 it"..... something along those lines. But sometimes, it might be under the five minutes. Why? Because in the morning times, things are different. There just aren't as many people online then as there are later in the day, or at night. And when I have cancelled two tourneys already due to not enough players, and I have 4 in at 3 past the hour, you betcha I'm gonna close it! It's better than waiting another two - 5 minutes in hopes another 4 will show up and lose a player because they are tired of waiting (which HAS happened, several times!)!
My point? It depends on the time of day, how many players have been and still are in the lobby and how many previous tourneys I've had to cancel as to how long I wait b4 Q4'ing a tourney.