NEW YORK — Four bolts — and their mysterious disappearance — are at the center of the MTA’s investigation into the Jan. 10 derailment of an F train in passenger service in Brooklyn, the Daily News has learned. MTA investigators are trying to track down the high-strength Grade 8 bolts, which were noticed missing from the undercarriage of the derailed car when crews arrived on the scene on Coney Island, transit sources told The News. The bolts are supposed to affix to a train’s undercarriage a piece called the “radius arm,” which helps keeps in place subway cars’ massive wheels and axles. Proper…