The string value of $TypesettingDim4 determines which symbols will be
displayed above 4-dimensional momenta,
Dirac matrices, metric tensors and polarization vectors. This concerns
only typesetting in the TraditionalForm output and doesn’t
change the physical behavior of those objects.
Overview, $TypesettingDimD, $TypesettingDimE.
$TypesettingDim4\_