Create a black frame on V2 and mask it using inverted track matte with a white frame on v3 that is 2880 pixels (3840*0.75) wide and 2160 tall. Nest the result
This will mask your 4×3 source layer on v1 that you can resize, independent of crop/mask, to fix blanking errors.
To see blanking errors more clearly, step into the nested sequence and add an effect to bring brightness up so the black bars are white. Turn off the effect before exporting to make it black again.
Beware that sim broadcasters will not accept black pillar boxing