I used to have a chart that gave the approximate times that food took in each part of the digestive system...but I can't find it for the life of me! When they're talking about 4 hours they may mean that in that time it will get broken down sufficiently so that nutrients begin to be absorbed...but there is no way that it passes through the entire GI tract in 4 hours...unless someone gets food poisoning or drinks a whopping dose of something like Epsom Salts that violently forces it through. There's about 30 feet of twisting and turning intestines alone...18 hours is very good...most people have a transit time of 24-36 hours and some even longer.