If it was recorded in 16:9, no there is no way to change this to 4:3 without cropping, letterboxing or deforming the image (which will result in things being the wrong shape).

Cropping or letterboxing will be your best options. Probably cropping as you would still get full screen but you would lose a bit on the left and right. You could move the viewport around though if there was something in the cropped bit that needed to be seen.

Howver I would not do this - I would just render it out at 16:9. If it is viewed on a TV, it would likely just show ithe middle and chop off the edges or letterbox it, depending on the TV. (Think about what happens when you watch a film on a 4:3 TV that is broadcast widescreen)