Hmm, four hours is a long time. It would be... 864000 seconds. Having it count down from that should work, I suppose, but it is a big number, and I don't like big numbers in programming / scripting. Having a variable set to 864000 and subtracting dt from it should work so long as there aren't any strange rounding errors or anything. I would give it a try with a stopwatch or something.
Edit: Someone give some authority on this; is using the wait function for large amounts of time a a bad idea? I would guess using wait(864000) would produce some problems, but it's a guess.