Under the Whyte notation for the classification of steam locomotives, a 4-8-4 locomotive has four leading wheels arranged in a leading truck, eight coupled driving wheels and four trailing wheels in a trailing truck.
The 4-8-4 was an obvious progression from the 4-8-2 "Mountain" and the 4-6-4 "Hudson" types, combining the 4-8-2's ability to have more weight on the driving wheels (leading to greater traction, and allowing a larger, more powerful locomotive) and the 4-6-4's larger firebox supported by a 4-wheel trailing truck, allowing for freer steaming, particularly at speed.
The first 4-8-4 was produced in 1927 by Alco for the Northern Pacific, and the type was therefore named "Northern". Southern railroads in particular obviously didn't find this name very suitable, so they chose other names for their 4-8-4 classes.
The Northern type proved to be the best choice of locomotive arrangement for both express passenger and fast freight service. It wasn't suited to heavy drag freight, but faster and lighter trains were well suited to the type.
Given that the 4-8-4 was a late development and Northerns were often the 'name' passenger power at the time of steam's demise, many were saved from the scrapper's torch and are now preserved in museums, or in the case of a lucky few, kept in running condition.
The Union Pacific has a Northern on its roster that has never been retired from service; class FEF-3 #844, the last steam locomotive built for the UP. It's used for charter service and for publicity for the railroad.