Everyone has already given you great advice.
As Walter said, if you have people in your shot they will look bizarre if squashed to fit a widescreen ratio. Sometimes you might not want the image to be pillar boxed and you may not want a graphic in the background.
In this scenario, another thing to try is to click the 4:3 clip in the sequence, select the ‘Motion’ tab and go to ‘Distort’ options. Set the aspect ratio to somewhere between 30 and 37. This will ‘widen’ your image a bit but not so noticeably and you can avoid pillar boxing or backdrop graphics, should you not want to use either of those work arounds. The downside is that you need to blow up the image slightly to compensate. So, if you set the aspect ratio to, 33, say, you’ll need to put the scale to 133.33 exactly to fill the entire frame.
Obviously blowing up any image is never ideal, but I’ve got acceptable results using this method on some occasions. Not perfect, but another option anyway!