  The second and fourth Fourier harmonics of the azimuthal distribution of
particles, $v_2$ and $v_4$, have been mesured in Au-Au collisions at the
Relativistic Heavy Ion Collider (RHIC). The ratio $v_4/(v_2)^2$ is
significantly larger than predicted by hydrodynamics. Effects of partial
thermalization are estimated on the basis of a transport calculation, and are
shown to increase the ratio by a small amount. We argue that the large value of
$v_4/(v_2)^2$ seen experimentally is mostly due to elliptic flow fluctuations.
However, the standard model of eccentricity fluctuations is unable to explain
the large magnitude of $v_4/(v_2)^2$ in central collisions.
