Use a 4:3 sequence and scale the footage down so that you see all of the image, but none of the bars. Of, use a 16:9 sequence and crop to 4:3 on export.
But HOW to crop to 4x3 for export?
On the Export Settings' Source tab, click the Crop button; you can select 4:3 from the Crop Proportions dropdown. This will shove the crop way over to one size, but you can adjust the size and position of the crop as needed to center it up.
Once you pick your Format and Preset (or tweak as you like for 4:3 output), toggle back and forth between the Source and Output tabs to make sure you're getting a clean crop. Once you have it how you like it, save yourself some trouble and create a new preset, which you can then apply to as many clips as you like.