undiscovered $z=0.06$ cluster in the Early Data Release of the SDSS detected by the C4 algorithm. The black dots are galaxies within an aperture of $1h^{-1}$Mpc around the cluster center. The red and green dots are actual cluster members (within $4\sigma_v$ in redshift space). The red points are galaxies with a low $H_{\alpha}$ equivalent width, while green points are galaxies with a high $H_{\alpha}$ equivalent width, [*i.e.*]{}, passive and star–forming galaxies respectively. The errors on the colors are shown for the red points, indicating the typical error bars on colors within the SDSS spectroscopic sample. Note the tight correlation in color of the red points, which is the E/S0 ridge line and is the signal the C4 algorithm is using to find clusters.[]{data-label="cm"}](fourplot.ps){width="9cm"}

### Overview of C4 Algorithm

I present here a brief overview of the C4 algorithm. To date, the C4 algorithm has been applied to the SDSS main galaxy spectroscopic sample (Strauss et al. 2002) in the Early Data Release (EDR) of the SDSS (see Stoughton et al. 2002; G[' o]{}mez et al. 2003).

For each galaxy in the sample – called the “target” galaxy below – the C4 algorithm is performed in the following steps:

1.  A 