Make sure your project is set for 4:3. Open Pan/Crop and UNcheck the option on the left side for "maintain aspect ratio". Right-mouse-button click inside the cropping frame and choose "match output aspect". (Note that there are actually two of these: one is a button near the top that allows you to change the shape of the cropping frame; this is not the one you want. The one you want is farther down and actually has the words "maintain output aspect".)

And of course, when you render, make sure you choose a 4:3 template. Just to be safe there's also an option there for something like "stretch image to fit" that you can check.