Simple solution - if the show ends at 4:00 just change it to 6:00.
Everyone will scoot out "early" at 4:00 and nobody's the wiser. :)
Though I do think your idea is a good one. We do plenty of 4h shows with time restrictions. I wouldn't mind some kind of incentive for voluntarily staying to the end.
Describe the prize that way, as an incentive to get people to stay the full time, and it'll help everyone understand that it's important to stick around.
Much better than a mandate stating "you HAVE to stay, or else".