  We present new arcminute-resolution maps of the Cosmic Microwave Background
temperature and polarization anisotropy from the Atacama Cosmology Telescope,
using data taken from 2013-2016 at 98 and 150 GHz. The maps cover more than
17,000 deg$^2$, the deepest 600 deg$^2$ with noise levels below 10
$\mu$K-arcmin. We use the power spectrum derived from almost 6,000 deg$^2$ of
these maps to constrain cosmology. The ACT data enable a measurement of the
angular scale of features in both the divergence-like polarization and the
temperature anisotropy, tracing both the velocity and density at
last-scattering. From these one can derive the distance to the last-scattering
surface and thus infer the local expansion rate, $H_0$. By combining ACT data
with large-scale information from WMAP we measure $H_0 = 67.6 \pm 1.1$
km/s/Mpc, at 68% confidence, in excellent agreement with the
independently-measured Planck satellite estimate (from ACT alone we find $H_0 =
67.9 \pm 1.5$ km/s/Mpc). The $\Lambda$CDM model provides a good fit to the ACT
data, and we find no evidence for deviations: both the spatial curvature, and
the departure from the standard lensing signal in the spectrum, are zero to
within 1$\sigma$; the number of relativistic species, the primordial Helium
fraction, and the running of the spectral index are consistent with
$\Lambda$CDM predictions to within $1.5 - 2.2\sigma$. We compare ACT, WMAP, and
Planck at the parameter level and find good consistency; we investigate how the
constraints on the correlated spectral index and baryon density parameters
readjust when adding CMB large-scale information that ACT does not measure. The
DR4 products presented here will be publicly released on the NASA Legacy
Archive for Microwave Background Data Analysis.
