  We limit the possible contributions from non-cosmological sources to the
COBE-DMR four-year sky maps. The DMR data are cross-correlated with maps of
rich clusters, extragalactic IRAS sources, HEAO-1 A-2 X-ray emission and 5 GHz
radio sources using a Fourier space technique. There is no evidence of
significant contamination by such sources at an rms level of ~8 uK (95%
confidence level at 7 degree resolution) in the most sensitive 53 GHz sky map.
This level is consistent with previous limits set by analysis of earlier DMR
data and by simple extrapolations from existing source models. We place a 95%
confidence level rms limit on the Comptonization parameter averaged over the
high-latitude sky of $\delta y\ <\ 1\ \times\ 10^{-6}$. Extragalactic sources
have an insignificant effect on CMB power spectrum parameterizations determined
from the DMR data.
