Molly Merchants is the true hidden main villain in Home Alone 4.
Molly is introduced as the housekeeper for Peter McCallister (father of main protagonist Kevin McCallister) and his fiancee, Natalie Kalban. She is originally portrayed as a sweet and kind woman to Kevin, often protecting him from curmudgeonly butler Prescott. After Marv Merchants and his wife, Vera, got into the home with plans to kidnap a Prince from the Royal Family, Kevin confided in Molly that Prescott could be working with them, as he heard Marv mention an "inside man." Molly dismissed Kevin's statements by stating that he had a wild imagination.
On the day of the Royal Family's arrival (the film's climax), Kevin planned for Marv and Vera's appearance and locked Prescott in the basement, still believing him to be the inside man. He later approached Molly and informed her about his plan and how he subdued Prescott, but at that moment, Molly turned heel and revealed to Kevin that she, not Prescott, was the inside man. She was also revealed as Marv's mother (as well as the true villainous mastermind behind the kidnapping plot) shortly after he and Vera arrived, and she later took Kevin and locked him in the basement with Prescott, cementing Molly's heel turn.
After the pair escaped, the villainous trio went after Kevin, with one of the results seeing Molly trapped in the elevator by Kevin, while still ordering her son and daughter-in-law to subdue him. After Vera and Molly were defeated, the evil Molly escaped from the elevator and attempted to knock out Kevin with a frying pan, only to be clocked herself by Prescott. The film ends with Molly being arrested and taken away in handcuffs, along with Marv and Vera.