I've just spent (or wasted) 4 hours trying a four deck solution with VDJ.
First thing to note, you require two copies of the program to be running at the same tme on the same machine (make sure you have lots of free RAM)
Second you need to download one of the "community created" skins for multi-instance playback. DO NOT switch them into full screen, they have to be in windowed mode (i don't like that at all ...hmmmph !)
This is where it all goes wrong. I could not for the life of me get decent sound/latency and beat lock at the same time. When running multiple decks the beat lock is number one on the requirements list. I have spent hours trying to configue this system to work and it's on a monster PC quad cores with 16Gb DDR2 1066 memory and raptor disks & Audio 8 / fast track pro, or sapphire pro, and it wouldn't play properly, as soon as I switch to the second instance of VDJ the beatlock slows down the playback and sounds terrible. I haven't even got to the timecode issues yet ! This is before I port it over to run on my live laptop
My advice for those who want a four deck solution, turn the VDJ software into a nice coaster and get Traktor, yes you have to spend more money , but it's better than wasting hours trying to get decks to sync. Traktor works directly out of the box, VDJ needs a lot of changing settings, loading patches/skins etc ... It might work for someone else, but this is not what I need in a "live environment".