Changing Screen Size on Samsung HP-R4272

When I hook up my high definition cable box to my Samsung plazma with either an HDMI cable or a Component Cable the high definition chanels all look terrific, but the chanels that aren't high definition are permanently shown in 4:3, and can't be "stretched" with the picture size funtion built into the television.  The only way I'm able to "stretch" the picture of chanels that aren't high definition (so they cover the entire screen) is to hook the cable box to the television using standard AV cables, which results in significant degredation of the picture quality when watching a high definition channel.  The television instruction manual states that when watching a high defintion channel through an HDMI or Component connection, only 16:9 and 4:3 will work (not panoramic, or either stretch function).  But, when a non high definition feed is coming through an HDMI or Component connection, shouldn't I be able to use those features?  I'm worried about burn-in if I watch a 4:3 picture for too long on this plazma television