  Symmetry breaking governs most fascinating phenomena in crystals, such as
ferroelectricity, nonlinear optics, piezoelectricity, ferromagnetism, and
superconductivity. In two-dimensional materials, a wide variety of tuning knobs
presents extraordinary opportunities for engineering symmetry breaking, leading
to the emergence and manipulation of novel physical properties. Recently,
tetralayer graphene with ABCB stacking order is predicted to possess atypical
elemental ferroelectricity arising from the symmetry breaking induced by its
specific stacking configuration. Experimentally unveiling the stacking-order
dependent symmetry in tetralayer graphene is crucial to understand the
intricate properties in the emergent graphene allotropes. Here, we observe
pronounced nonlinear optical second harmonic generation (SHG) in ABCB-stacked
tetralayer graphene, but absent in both ABAB- and ABCA-stacked allotropes. Our
results provide direct evidence of symmetry breaking in ABCB-stacked tetralayer
graphene. The remarkable contrast in the SHG spectra of tetralayer graphene
allows straightforward identification of ABCB domains from the other two kinds
of stacking order and facilitates the characterization of their crystalline
orientation. The employed SHG technique serves as a convenient tool for
exploring the intriguing physics and novel nonlinear optics in ABCB-stacked
graphene, where spontaneous polarization and intrinsic gapped flat bands
coexist. Our results establish ABCB-stacked graphene as a unique platform for
studying the rare ferroelectricity in non-centrosymmetric elemental structures.
