  The cosmic microwave background radiation provides unique constraints on
cosmological models. In this Letter we present a summary of the spatial
properties of the cosmic microwave background radiation based on the full 4
years of COBE DMR observations, as detailed in a set of companion Letters. The
anisotropy is consistent with a scale-invariant power law model and Gaussian
statistics. With full use of the multi-frequency 4-year DMR data, including our
estimate of the effects of Galactic emission, we find a power-law spectral
index of $n=1.2\pm 0.3$ and a quadrupole normalization
$Q_{rms-PS}=15.3^{+3.8}_{-2.8}$ $\mu$K. For $n=1$ the best-fit normalization is
$Q_{rms-PS}\vert_{n=1}=18\pm 1.6$ $\mu$K. These values are consistent with both
our previous 1-year and 2-year results. The results include use of the $\ell=2$
quadrupole term; exclusion of this term gives consistent results, but with
larger uncertainties. The 4-year sky maps, presented in this Letter, portray an
accurate overall visual impression of the anisotropy since the signal-to-noise
ratio is ~2 per 10 degree sky map patch. The improved signal-to-noise ratio of
the 4-year maps also allows for improvements in Galactic modeling and limits on
non-Gaussian statistics.
