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We calculate the fourth cluster coefficients of the homogeneous unitary spin1/2 Fermi gas as functions of the internal-state mass ratio, over intervalsconstrained by the 3- or 4-body Efimov effect. For this we use our 2016conjecture (validated for equal masses by Hou and Drut in 2020) in anumerically efficient formulation making the sum over angular momentum convergefaster, which is crucial at large mass ratio. The mean cluster coefficient,relevant for equal chemical potentials, is not of constant sign and increasesrapidly close to the Efimovian thresholds. We also get the fourth virialcoefficients, which we find to be very poor indicators of interaction-induced4-body correlations. We obtain analytically for all $n$ the clustercoefficients of order $n$ + 1 for an infinity-mass impurity fermion, matchingthe conjecture for $n=3$. Finally, in a harmonic potential, we predict anon-monotonic behavior of the 3 + 1 cluster coefficient with trappingfrequency, near mass ratios where this coefficient vanishes in the homogeneouscase.

Fourth cluster and virial coefficients of a unitary Fermi gas for an arbitrary mass ratio