Is 4-channel mixing of videos possible?

No, but 2-channel mixing is possible.

With a 4-deck layout, the decks whose videos are to be mixed are set on the computer screen (decks [A] and [B] by default). The two decks whose indicators ([A] to [D]) displayed on the sides of the [VIDEO CROSSFADER] control on the Video panel are enabled are the decks whose videos will be mixed.