A 4-4-6-4, in the Whyte notation for the classification of steam locomotives by wheel arrangement, is one that has four leading wheels followed by four coupled driving wheels, a second set of six coupled driving wheels and four trailing wheels.

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The Pennsylvania Railroad's Q2 class were the only locomotives ever to use this arrangement. These were duplex locomotives, in which both sets of driving wheels were mounted in a common, rigid locomotive frame. This locomotive design was a further development of the highly successful 2-10-4. The divided drive, or duplex arrangement, allowed for higher speeds with less damage to the track.

A proposed design for the Lehigh Valley Railroad was done but it was never built.^[1]

Other equivalent classifications are:
UIC classification: 2BC2 (also known as German classification and Italian classification)
French classification: 2232
Turkish classification: 2435
Swiss classification: 2/4+3/5 up to the early 1920s, later 5/9