  In quantum many-body systems, particularly, the ones with large near-EF
density states, like flat bands or van Hove singularity (VHS), electron
correlations often give rise to rich phase diagrams with multiple
coexisting/competing orders occurring at similar energy scales. The recently
discovered locally noncentrosymmetric heavy fermion superconductor CeRh2As2 has
stimulated extensive attention due to its unusual H-T phase diagram, consisting
of two-phase superconductivity, antiferromagnetic order, and possible
quadrupole-density wave orders. However, despite its great importance, the
near-EF electronic structure remains experimentally elusive. Here, we provide
this key information by combining soft X-ray and vacuum ultraviolet (VUV)
angle-resolved photoemission spectroscopy measurements and atom-resolved DFT+U
calculations. With bulk-sensitive soft X-rays, we reveal quasi-2D hole- and 3D
electron- pockets with a pronounced nesting feature. Most importantly, we
observe a symmetry-protected fourfold VHS coexisting with the Ce 4f flat bands
near the EF, which, to the best of our knowledge, has never been reported
before. Such a rare coexistence is expected to lead to a large density of
states at the zone edge, enhancement in electron correlations, and a large
upper critical field of the odd-parity superconducting phase. Uniquely, it will
also result in a new type of f-VHS hybridization that alters the order and fine
electronic structure of the symmetry-protected VHS and flat bands. These
peculiarities offer important dimensions for understanding the reported rich
phase diagram and are discussed as an origin of superconductivity with two
phases. Our findings not only provide key insights into the nature of multiple
phases in CeRh$_2$As$_2$, but also open up new prospects for exploring the
novelties of many-body systems with f-VHS hybridization.
