Hey Christopher,
If you wanted to use 4 turntables for DVS, you'd have to use a hardware interface which supports 4 turntable inputs like the DJM-900NXS2. If you only wanted to use 2 of them for DVS, then there is a larger variety of hardware options. Unfortunately you can't "split" the DVS signal to control two different software decks, stereo signal is required for the input.
What you could do, again, depending on the hardware you're using, is perform an "instant double" to move the song that was on deck 1 to deck 3, then use DVS to control deck 1 while the song plays on deck 3 without control.
You could also use 2 computers and 2 interfaces, routing outputs from each into a mixer. You couldn't use 1 interface for two computers with 4 turntables (again, unless using a device which supported it, like the 900NXS2 above).
If using the DJM-900NXS2, you could use all the listed software, but with other devices (like the Interface2), it would be restricted to rekordbox. You'd need a Club Kit license for Serato, a Core subscription for rekordbox, but only the software license for Traktor.
I'm sure you'll have other questions, so hit me with 'em!