If you want to keep the aspect ratio, the only way to get a full 4:3 Screen is to cut left and right, but you will lose much Picture informations.
In tmpgenc you can make it in this way:
At "Video arrange Method" select "Full screen(keep aspect ratio)"

Mark the Clip Frame Filter and doubleclick it, do get to its settings.

In this Window, mark the "Resized image is used to display preview" field, that you can see the results of your Changes directly.
Now you can raise the "right" and "left" values until the picture is Fullscreen.
as i said, you will lose much Picture informations
Good Luck