I have run three lairs out of the pack (Pit of the Dust Goblins, Crypt of Green Shadows, and Carrion Shrine of Qorgeth) and none of them were done in four hours. With the traps and branching paths, there are a lot of choices for the players, which leads to a fair amount of discussion (which door should we tackle first, etc.). Most of the encounters are also pretty tough for the average party level, if you play the monsters right. In sum, I'd say one of the above adventures should at last well over four hours.