  We present a new determination of the cluster mass function and velocity
dispersion function in a volume $\sim10^7 h^3$Mpc$^{-3}$ using the Fourth Data
Release of the Sloan Digital Sky Survey (SDSS). We use the caustic technique to
remove foreground and background galaxies. The cluster virial mass function
agrees well with recent estimates from both X-ray observations and cluster
richnesses. The mass function lies between those predicted by the First-Year
and Three-Year WMAP data. We constrain the cosmological parameters $\Omega_m$
and $\sigma_8$ and find good agreement with WMAP and constraints from other
techniques. With the CIRS mass function alone, we estimate
$\Omega_m=0.24^{+0.14}_{-0.09}$ and $\sigma_8=0.92^{+0.24}_{-0.19}$, or
$\sigma_8=0.84\pm$0.03 when holding $\Omega_m=0.3$ fixed. We also use the WMAP
parameters as priors and constrain velocity segregation in clusters. Using the
First and Third-Year results, we infer velocity segregation of
$\sigma_{gxy}/\sigma_{DM}\approx0.94\pm$0.05 or 1.28$\pm$0.06 respectively. We
compare the velocity dispersion function of clusters to that of early-type
galaxies and conclude that clusters comprise the high-velocity end of the
velocity dispersion function of dark matter haloes. The evolution of cluster
abundances provides constraints on dark energy models; the mass function
presented here offers an important low redshift calibration benchmark.
