  We numerically simulate the free decay of two-dimensional quantum turbulence
in a large, homogeneous Bose-Einstein condensate. The large number of vortices,
the uniformity of the density profile and the absence of boundaries (where
vortices can drift out of the condensate) isolate the annihilation of
vortex-antivortex pairs as the only mechanism which reduces the number of
vortices, $\Nv$, during the turbulence decay. The results clearly reveal that
vortex annihilation is a four-vortex process, confirming the decay law $\Nv
\sim t^{-1/3}$ where $t$ is time, which was inferred from experiments with
relatively few vortices in small harmonically trapped condensates.
