SI[mu]
can be used as input for -dimensional with 4-dimensional Lorentz index and is transformed into PauliSigma[LorentzIndex[mu]]
by FeynCalcInternal.
Overview, PauliSigma, SID, SIE.
[\[Mu]] SI
[\[Mu]] // FCI // StandardForm
SI
(*PauliSigma[LorentzIndex[\[Mu]]]*)
[\[Mu], \[Nu]] - SI[\[Nu], \[Mu]] SI
[\[Mu], \[Nu], \[Rho], \[Sigma]] SI
[\[Mu], \[Nu], \[Rho], \[Sigma]] // StandardForm
SI
(*SI[\[Mu]] . SI[\[Nu]] . SI[\[Rho]] . SI[\[Sigma]]*)
[\[Alpha]] . (SIS[p] + m) . SI[\[Beta]] SI