Somewhere in the settings, there's usually an "Automatic".
You'll even hear the thing go "tick" and switch between 4:3 and 16:9 when the adverts come on.
If the set is "16:9" (and most are, these days), you can also tell it with 4:3 whether to leave empty bands each side of the picture, chop the top off, or half-way-between.