  We measure the weak-lensing masses and galaxy distributions of four massive
galaxy clusters observed during the Science Verification phase of the Dark
Energy Survey. This pathfinder study is meant to 1) validate the DECam imager
for the task of measuring weak-lensing shapes, and 2) utilize DECam's large
field of view to map out the clusters and their environments over 90 arcmin. We
conduct a series of rigorous tests on astrometry, photometry, image quality,
PSF modeling, and shear measurement accuracy to single out flaws in the data
and also to identify the optimal data processing steps and parameters. We find
Science Verification data from DECam to be suitable for the lensing analysis
described in this paper. The PSF is generally well-behaved, but the modeling is
rendered difficult by a flux-dependent PSF width and ellipticity. We employ
photometric redshifts to distinguish between foreground and background
galaxies, and a red-sequence cluster finder to provide cluster richness
estimates and cluster-galaxy distributions. By fitting NFW profiles to the
clusters in this study, we determine weak-lensing masses that are in agreement
with previous work. For Abell 3261, we provide the first estimates of redshift,
weak-lensing mass, and richness. In addition, the cluster-galaxy distributions
indicate the presence of filamentary structures attached to 1E 0657-56 and RXC
J2248.7-4431, stretching out as far as 1 degree (approximately 20 Mpc),
showcasing the potential of DECam and DES for detailed studies of degree-scale
features on the sky.
