Well, it does and it doesn't. I think that the default is to fill the screen when you drop in a 4:3 clip, so it zooms in to do that and you loose some of the clip. However, if you click on the Adjust button, above the Preview window, then the Crop button, you can choose "Fit" where you'll get your full 4:3 clip with black borders at each side.
This is no bad thing because if you make a dvd or view the resultant movie file, it will go to Fullscreen without any distortion.
If you still have iMovie6 HD, do you still have iMovie11? That will produce full 4:3 Projects.