  Quartet superfluid (QSF) is a distinct type of fermion superfluidity that
exhibits high-order correlation beyond the conventional BCS pairing paradigm.
In this Letter, we report the emergent QSF in 2D mass-imbalanced Fermi mixtures
with two-body contact interactions. This is facilitated by the formation of
quartet bound state in vacuum that consists of a light atom and three heavy
fermions. For an optimized heavy-light number ratio $3:1$, we identify QSF as
the ground state in a considerable parameter regime of mass imbalance and 2D
coupling strength. Its unique high-order correlation can be manifested in the
momentum-space crystallization of pairing field and density distribution of
heavy fermions. Our results can be readily detected in Fermi-Fermi mixtures
nowadays realized in cold atoms laboratories, and meanwhile shed light on
exotic superfluidity in a broad context of mass-imbalanced fermion mixtures.
