  We present an extension to a Sunyaev-Zel'dovich Effect (SZE) selected cluster
catalog based on observations from the South Pole Telescope (SPT); this catalog
extends to lower signal-to-noise than the previous SPT-SZ catalog and therefore
includes lower mass clusters. Optically derived redshifts, centers, richnesses
and morphological parameters together with catalog contamination and
completeness statistics are extracted using the multi-component matched filter
algorithm (MCMF) applied to the S/N>4 SPT-SZ candidate list and the Dark Energy
Survey (DES) photometric galaxy catalog. The main catalog contains 811 sources
above S/N=4, has 91% purity and is 95% complete with respect to the original
SZE selection. It contains 50% more total clusters and twice as many clusters
above z=0.8 in comparison to the original SPT-SZ sample. The MCMF algorithm
allows us to define subsamples of the desired purity with traceable impact on
catalog completeness. As an example, we provide two subsamples with S/N>4.25
and S/N>4.5 for which the sample contamination and cleaning-induced
incompleteness are both as low as the expected Poisson noise for samples of
their size. The subsample with S/N>4.5 has 98% purity and 96% completeness, and
will be included in a combined SPT cluster and DES weak-lensing cosmological
analysis. We measure the number of false detections in the SPT-SZ candidate
list as function of S/N, finding that it follows that expected from assuming
Gaussian noise, but with a lower amplitude compared to previous estimates from
simulations.
