  We investigate the form of the viscous correction to the equilibrium
distribution function in the context of a Cooper-Frye freeze out prescription
for viscous hydrodynamic simulations of relativistic heavy ion collisions. The
standard quadratic ansatz used by all groups for the case of shear viscosity is
found to be disfavored by experimental data for v4/(v2)^2 at the Relativistic
Heavy Ion Collider, and is unlikely to be correct for the hadron resonance gas
present at freeze out. Instead, data for v2(pt) along with v4/(v2)^2 favor a
momentum dependence between linear and quadratic.
