We present a spectroscopic sample of 747 detached close binary systemsfrom the Sloan Digital Sky Survey (SDSS) Fourth Data Release. Themajority of these binaries consist of a white dwarf primary and alow-mass secondary (typically M dwarf) companion. We have determined thetemperature and gravity for 496 of the white dwarf primaries and thespectral type and magnetic activity properties for 661 of the low-masssecondaries. We have estimated the distances for each of the whitedwarf-main-sequence star binaries and use white dwarf evolutionary gridsto establish the age of each binary system from the white dwarf coolingtimes. With respect to a spectroscopically identified sample of ~8000isolated M dwarf stars in the SDSS, the M dwarf secondaries showenhanced activity with a higher active fraction at a given spectraltype. The white dwarf temperatures and gravities are similar to thedistribution of ~1900 DA white dwarfs from the SDSS. The ages of thebinaries in this study range from ~0.5 Myr to nearly 3 Gyr (average age~0.20 Gyr).