I suspect this was for a switch to operate the door opener. Those are low voltage circuits and even if it were still life you'd have little to worry about with it.

Low voltage power limited control circuits are not a hazard.

It is the same class as thermostat wiring and requires little protection since it is not capable of starting a fire.

My son said there is no way this 4 colored wire could carry in his words "house current 120 volts".... is that right?

And yes ronnatalie was right, it was once connected to a switch to operate the door on both sides. It was disconnected about 3 years ago and has been left dangling on my side. I havent confirmed if its connected still on the otherside.

If it was still live, how much voltage would be running thru it? Would it be AC or DC? And how would one test it with a multimeter?

My son said he didnt bother bringing his "multimeter" to test.

Around 24 VAC and only 1-2 V-A.

That is why control wiring does not require junction boxes for splices.