What kind of four wire connector is this?
Audio would make sense since the wires are white-black-white-black. That would match up to the two speakers you talked about.

Any line-voltage wires for switches or dimmers would have to be in an electrical box, so that leaves us with low-voltage and media cables as options (or a very bad electrician!).

I was looking around at home audio equipment and I noticed that 4-conductor in-wall speaker wire follows the same color pattern of Red-black-green-white that your connector has.

Although I don't know exactly what this connector is for, home audio seems very, very likely.