Factual error: Anytime a character fires a missile at an airborne target, it's invariably from an M136 antitank launcher. The M136 is designed to hit relatively slow moving ground vehicles and is useless against fast moving airborne targets. (This is because used M136 tubes can't be reloaded and are very cheap to use as props).
Factual error: In episode 17 there are some scenes showing sick 4400s all around the world. One of them is being carried into an ambulance car in Frankfurt, Germany. The ambulance car's number plate begins with the letters FM meaning Frankfurt am Main. The correct abbreviation for Frankfurt am Main on German number plates is F - not FM.