Originally Posted by planemechanic
8146 would usually be the nose number of the aircraft. The fleet is 8xxx for a 747 and the other three numbers are used to identify a subfleet.
Just a note to that, UA's 747-400OB are using 81xx, all the numbers are running from 8171 (N171UA) to 8128 (N128UA), therefore 8146 does not exist in UA's fleet at all.