You can "pillarize" your video (did I just coin a new term?) with MPEG Streamclip. Drop your 4:3 video in the app and select what format you'd like to export it to. Let's say, for qualities sake, you choose "Export to QuickTime" and select Apple DVCPRO50. Select the "FRAME SIZE" you want, which will most likely be 1280 x 720 or 1920 x 1080 if you're striving for 16:9. Interlaced scaling will be pre-selected by default, and you may as well leave it selected. Just for the **** of it select "Better Downscaling." Can't hurt...supposedly improves the quality of conversions, though the conversions take slightly longer.
Okay, on to the pillars. One of the adjustment selections you'll see is "ZOOM," which changes the aspect ratio of the video. Leave the first setting where it is at 100%. It's the next setting called "X/Y" where the pillar/letterbox action is. You'll notice X/Y offers three choices. The first, by default, is "1." That simply leaves the video at the mercy of the frame size you've selected - meaning that it would stretch your 4:3 into a 16:9 video. Not what you want. The next choice is "1.333." That'll letterbox your video, which is great for converting 16:9 to 4:3. It's the last choice offered that'll give you pillars - "0.75." After selecting it click the "Preview" button to see if the adjustment takes you where you want to go. If it doesn't, than I've completely misunderstood your question! Sorry.
Incidentally, you can remove the pillars/letterboxing by changing the Zoom field from 100% to 133.3%, which would enlarge your picture to fit 16:9 without stretching it. Quality may suffer a bit, but it's worth experimenting with. (The Zoom "Center" feature is usually left at zero, unless you want to move the picture to the left or the right a bit).
