  In tetralayer graphene, three inequivalent layer stackings should exist,
however, only rhombohedral (ABCA) and Bernal (ABAB) stacking have so far been
observed. The three stacking sequences differ in their electronic structure,
with the elusive third stacking (ABCB) being unique as it is predicted to
exhibit an intrinsic bandgap as well as locally flat bands around the K points.
Here, we use scattering-type scanning near-field optical microscopy and
confocal Raman microscopy to identify and characterize domains of ABCB stacked
tetralayer graphene. We differentiate between the three stacking sequences by
addressing characteristic interband contributions in the optical conductivity
between 0.28 and 0.56 eV with amplitude and phase-resolved near-field
nano-spectroscopy. By normalizing adjacent flakes to each other, we achieve
good agreement between theory and experiment, allowing for the unambiguous
assignment of ABCB domains in tetralayer graphene. These results establish
near-field spectroscopy at the interband transitions as a semi-quantitative
tool, enabling the recognition of ABCB domains in tetralyer graphene flakes and
therefore, providing a basis to study correlation physics of this exciting
phase.
