If you can use 2 skulls to talk to each other by using the left and right channels will 4 skulls talk to each using 4 channels?
Short answer - yes. A normal audio player (such as out BooTunes, a CD player, iPod, etc) has one stereo output. A stereo output has two channels (a left and a right). So to make two skulls talk to each other you use a spliter cable to split the left and right so each can go to one of the two skulls. Simple enough.
To have 4 skulls talking at the same time you need to do the same thing to two audio players and have those players fire off at the exact same time. this is easiest using the BooTunes.