We are about to install a chandelier in our bathroom and I am having difficulty figuring out how to wire the light. There is an existing ceiling box pre-wired which is tied in with 4 can lights in our bathroom. One light switch controls all 4 can lights plus the wires in the ceiling box which I tested with my voltage tester.

When I removed the cap, I noticed 2 electrical cables inside the junction box (picture below). One cable is 12-3 and the other is 12-4. How do I wire the light fixture to these electrical cables? Do I only use one of the cables or do I need to use both cables? Any help would be greatly appreciated and thank you in advance.

 

You want the new light to operate on it's own switch? Do you already have a second switch installed? What does that control?
Determine what set of wires in the ceiling box are hot with the can lights switched off. Are there any?

 

No, we want the new light to operate with the light switch that turns the can lights on.
Both sets of wires in the ceiling box are hot when the can light switch is on. None of the wires in the ceiling box are hot when the switch that controls the can lights is off.

 

here are pictures of the light switch and wiring if that helps.

 

Welcome to the forums.

You have two cables there.... 12-2 NM-b w/ground and a 12-3 NM-b with ground.
The ground is understood to be there and is not counted in wiring counts.

That's an awful lot of wiring to be found in what looks like a pancake box. The live/hot/switched wires should have been covered with wire nuts. If those wires are switched.... check between each wire and ground. The switched hot will only be live when the switch is on. Do not use a non contact test pen to locate the proper wiring. Use a test meter or a two wire voltage probe.

On edit:
Based on the pictures..... it looks like you'll be using the 12-2 cable.
The 12-3 looks to be not connected in the switchbox.

 

Can't explain any better than that!