For motion video, may need to refresh imagem position at (n) x time, not to cut heads, for example.
Power Tools / Video Crop / Crop, does this
PIP Designer, also AMD-FX 8350 / 8GB DDR3
SSD SUV400S37240G / 2-HD WD 1TB
AMD Radeon R9 270 / AOC M2470SWD
Windows 7-64 / PD16 Ultimate

As PlayVideo points out, Ronk, you would need to do a lot of adjusting, because you would need to be zoomed in on the 4:3 video in order to fill the screen. Composition would be out the window, and you would invariably have people's heads cut off, important parts of scenery left out etc.

Letterboxing looks fine, at least you're getting the full image. Just like the opposite, letterboxing to squeeze a 16:9 image into a square 4:3 TV screen.

But if you want to experiment to see how much damage would be done, do this to run the test:

--Set the Preview Window at 50% so there's lots of space all around the video image. With one of your 4:3 clips selected, grab the corner of the dotted line box and make it larger. You can grab the sides too. Keep going to the X at the center to move the whole image so it's centered correctly.

Eventually you'll get the whole 16:9 screen filled, and the dotted lines will show you how much of your original image will be lost.

Do that, find out if it's what you really want to do.

RB

Keep in mind that unless your original 4:3 video clip is of high enough pixel count (resolution) the result in 16:9 reformatting may be disappointing in quality. Win 10, i7