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.

It can change fairly easily within the company to help designate different sub-fleets within a fleet. The most common change is during an interior upgrade/change in which the number of seats/galley/lavs changes. This is how the schedulers and many others know the difference between planes without having to look it up or trying to remember all of the changes as they come out of maintenance. There are other sub-fleet designators such as overwater 737's that used to have life rafts in some of the stow bins.

The number 8146 can also be a radio call sign used when the same flight number is in the air at the same time.

Hope that helps.