In the Season 4 opener, the owner of "Buster's Beach House" in Long Beach wants Charles to investigate claims that his executive chef is showing preferential treatment to the wait-staff, but that turns out not to be the only problem the restaurant faces.
The family-owned "Columbo's Italian Steakhouse" in Eagle Rock, California is suffering from a lack of harmony among staff members partly due to a cutthroat Employee-of-the-Month competition.
Suspicions that the staff of "Gauchos Village" in Glendale, Cal., took advantage of the fact that their boss was away recovering from a near-fatal accident are investigated by Charles and the team, who look into revenue discrepancies.
After complaints about the new executive chef at "Bistro Bordeaux" in Evanston, Illinois, Charles Stiles discovers that the problem is he's being sabotaged by other staff members.
The owner of "Big and Little's Restaurant", a Chicago BYOB eatery, contacts Charles Stiles to investigate rumors that some of his employees are illegally selling alcohol to customers.
The owner of "Cheesie's Pub and Grub" in Chicago is suspicious after several petite women easily complete an eating challenge, so he asks Charles and the team to investigate whether employees are bending the rules for certain customers.
Charles Stiles helps the owner of Bella Luna Café, in Chicago, uncover problems with its delivery service, after the owner receive numerous complaints from customers, who were being mistreated.
The owner of "Gigi Restaurant" in Philadelphia asks Mystery Diners to investigate possible discrimination, from his mother/manager of Gigi, to young customers.
Charles Stiles and his team are called to Philadelphia, after rumors of a night security guard being too rough with customers at Del Rossi's Cheesesteak Co.
A member of the team goes undercover, as a drag queen, to investigate whether a weekly trivia competition is rigged, which is what the cross-dressing customers of Philadelphia's "Venture Inn" have suggested to the concerned owner.
Marvin, the owner of Falls Taproom in Philadelphia, contacts Charles to help investigate an ongoing feud between his girlfriend/manager and best friend/customer.
The owner of a Texas restaurant chain, called "Cooper's Old Time Pit Bar-B-Que, suspects that one of his employees has divulged a secret family recipe to a rival business.