  We compute v4/(v2)^2 in ideal and viscous hydrodynamics. We investigate its
sensitivity to details of the hydrodynamic model and compare the results to
experimental data from RHIC. Whereas v2 has a significant sensitivity only to
initial eccentricity and viscosity while being insensitive to freeze out
temperature, we find that v4/(v2)^2 is quite insensitive to initial
eccentricity. On the other hand, it can still be sensitive to shear viscosity
in addition to freeze out temperature, although viscous effects do not
universally increase v4/(v2)^2 as originally predicted. Consistent with data,
we find no dependence on particle species. We also make a prediction for
v4/(v2)^2 in heavy ion collisions at the LHC.
