How do I crop a video file to the 4:3 size?

Use AVS Video Converter

1. download, install the program and open the file
2. select output format
3. click the Edit Profile button and set 640x480 or 800x600 in the Frame Size field
4. click Save and Close
5. click the Advanced button twice
6. select the Aspect Correction tab
7. select Original video aspect
8. set Crop Scale to 100
9. click Convert Now
that's all
I think that AVS Video Converter is the best. It supports virtually all video formats and has built-in DVD, VCD burning engine.