you crop the 4:3 section you want out of the 16:9 area by putting the 16:9 footage into a 4:3 timelaine, and cropping the sides off so that whats left is the desired 4:3 dimensions and framing. most NLES have a preset for doing this. then you can change your cropping .. more on one side.. less on the other.. as needed so that you have the framing you want.
Do you want the footage full screen or letterboxed? If it's full screen, Neil's suggestion should work fine.
For letterboxed 4:3, you won't have to crop your 16:9 footage, to have it within a 4:3 sequence.
Create a new 4:3 sequence, drop your clip into the sequence. Make sure, within the clip's item properties (right/cntrl + click), that the anamorphic option is checked. (dependent on if the anamorphic option was checked during capture)
Should work fine, without cropping, if you desire letterbox.