I put mine on 16:9 and leave it there. It goes full screen when a 16:9 broadcast is detected. I don't like the unnatural stretch of a 4:3 broadcast. I got used to the black bars rather quickly. My local digital networks are converting to full time 16:9 HD content except for most commercials. I'm watching far less standard definition in 4:3 these days.
It matters what kind of connection is used between the box and the set. Some set inputs restrict the availability of certain features. Component video cables (Y, Pr, Pb) or HDMI is the proper connection method. Your old cable box may or may not offer either. Try bypassing the box with the cable for now.