LorentzIndex[mu] denotes a 4-dimensional Lorentz index.
For other than 4 dimensions: LorentzIndex[mu, D] or LorentzIndex[mu] etc.
LorentzIndex[mu, 4] simplifies to LorentzIndex[mu].
Overview, ChangeDimension, Momentum.
This denotes a 4-dimensional Lorentz index.
LorentzIndex[\[Alpha]]\alpha
An optional second argument can be given for a dimension different from 4.
LorentzIndex[\[Alpha], n]\alpha