If you're watching a 4x3 program on a 16x9 TV set, you're going to have black bars on the sides, the picture just doesn't fit the screen. If you really don't want the black bars, you can stretch the screen, which distorts the image, or zoom, which doesn't distort, but cuts off the top and bottom of the picture. Your choice.
I do dislike a 16x9 movie jammed in a 4x3 area. There are some Netflix movies that have this, and some SD TV channels do this. I've found that in this case, zoom doesn't seem to do anything, but cinema works. For example, I have digital cable, and if I watch the <s>Sci-Fi</s> SyFy HD channel, all is fine. If I watch the SD SyFy channel, then it is a 16x9 movie jammed into a 4x3 area. One thing I don't like about cable is that the channels are chaotically layed out, the HD and the SD SyFy channel are far, far apart on the listing, with the SD channel being at 44, while the HD channel is at 400+.