I have installed several of the Lutron Diva dimmer switches in my house with no problem. Like the others, the switch I am replacing is a 3 way. However instead of three wires plus ground on the original switch, there are 4 wires plus ground. There are 2 black wires and 2 red wires. One black and one red are connected to brass screws at the top of the switch and the other black and other red are connected to dark colored screws at the bottom of the switch. I am assuming that two of the existing wires will be connected to a single wire on the Diva dimmer, but I am not sure how to determine which two?