Currently double-clicking on a track somewhere in the library adds it to the first non-playing deck. However, when four-decks-mode is activated and both of the two original decks have tracks in them, even if one isn't playing, double-clicking a third track in library merely switches out the second track from the non-playing second deck. Decks two and three remain empty unless tracks are manually dragged to them; double-clicking will never fill them.
It would be intuitive if decks got filled to capacity, i.e. as long as there is an empty deck, double-clicking a track will get that track sent to one of them.
Rapid-fire deck loading seems pretty cool. Though due to the way crossfading works the DJ would have to be careful what order they double-clicked the tracks in so that the specific tracks they wanted could get faded between the targeted decks.
This is even more pronounced than I thought; when in four-decks mode, and the playing track is in deck 3 or 4, and decks 1 and 2 are empty, double-clicking on a track in library loads to deck 1 and only to deck 1. Double-clicking on another track just switches the track out of deck 1.
Bug title changed accordingly.