Schematic for dual fixtures/wiring
The basic wiring would look something like this.



Since code requires a grounded (neutral) conductor at each switch, you'll have to carry it through all the way to the last switch.  Because of this, the cables between the switches will need an extra conductor.  

You'll have to pull 14/3 with ground between the light and the first switch box. Then you'll pull either 14/4 with ground if you can find it, or 14/2/2 with ground between each switch location .

NOTE: If this is a 20 ampere circuit, use 12 AWG conductors instead of 14 AWG.



The first and last switches will be 3-way, while the two in the middle will be 4-way switches.



Adding more lights is as simple as tapping off of the wires that connect to the first light.



NOTE: Grounding conductors are not shown in the diagrams, make sure all devices and enclosures are properly grounded.