The Numark Mixdeck Quad supports the use of two virtual decks within Serato DJ. The remaining two channels on the controller can be used as external hardware channels. 

Due to the configuration of the controllers internals, it is unable to be used as a 4-channel controller in Serato DJ. The Mixdeck Quad can still use 4 channels of audio, however the other 2 tracks will need to have come in another form of media (CD, USB, External Input etc).