Virial expansion is widely used in cold atoms to analyze high
temperature strongly correlated many-body systems. As the n-th order
virial expansion coefficient can be accurately obtained by exactly
solving up to n-body problems, the virial expansion offers a few-body
approach to study strongly correlated many-body problems. In particular,
the virial expansion has successfully been applied to unitary Fermi gas.
We review recent progress of the virial expansion studies in the unitary
Fermi gas, in particular the fourth order virial coefficient.