AVID - Creating 4:3 look in a 16:9 4096 x 2160 or similar Avid Project

Go into Effects Mode

Search for the Mask Effect in the Effects PaletteĀ 
on the timeline in AVID make a new video track and drag the Mask (Image)
effect to the upper track with filler loaded

Then go into effect mode
change the width scaling to 75