By your title, it sounds like you want to stretch from a square pixel to a widescreen pixel, 4:3 to 16:9, there would be no problem, just black on the sides.
Is your footage squished or stretched and what is it supposed to be? 4:3 is square, so if I assumed that you accidently switched the ratios backwards and were trying to go from widescreen to square, you would have to choose from these 3 professional approaches to this common squished look problem.
-Variable
Fit the input image to the output size. If the
input and output resolutions do not have the
same aspect ratio, the output image is stretched.
————————————————–
-Crop
Fit the input image into the output resolution
without distorting it. If necessary, the input
image is cropped to preserve the aspect ratio.
————————————————–
-Pad
Fit the input image into the output resolution
without distorting it. If necessary, the aspect
ratio is preserved by surrounding the output
image with a black border.