Trust Metric is the first episode of the fourth season. It was released on 28 September 2007.
Peter MacNicol returns as a regular cast member. Colby is being interrogated by a man named Agent Kirkland, who passes him a handcuff key and gives him escape instructions. Don and his team get information about and from Colby and must figure out if that information is reliable then realizes that everything about Colby being a double agent may not be what it seems.