Click on the little square on the clip to get into pan/crop. Use the preset drop-down and pick the 4:3 preset. Then, grab one of the little squares on the edge of the crop dotted line and move the cropping in so that the top edges are on the top edges of the picture.
Once again, thanks alot David, I'll try that. One more question: In the preview window there are many selections: Draft/Preview/Good/Best w/ the Full or Auto selections. Which one should I be using if I am going to 4:3. Or does it matter?
Best, especialy with a lot of FX drags/skips on me. Besides, if you're just previewing, doesn't really matter what you have it set at. Keep in mind, when you zoom in to get rid of the letterboxes, you're going to have a reduced video quality.