We present the fourth data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which was recently released and is publicly available. The data set consists of of time-of-arrival (TOA) measurements and timing models for 47 millisecond pulsars observed between 2004 and 2017 at the Arecibo Observatory and the Green Bank Telescope. This “12.5-year data set” comes in two varieties. The first, a “narrowband” data set, in which TOAs are measured from many narrow radio-frequency bands for every observation, is very similar in construction and form as our earlier data sets. The second, a “wideband” data set, in which a single TOA and a simultaneous estimate of the dispersion measure (DM) comprise the measurement from each of the broadband observations, is the first pulsar timing array (PTA) data set of its kind. A number of improvements in processing the raw profile data and the TOAs led to a cleaner and more uniformly constructed data set. We report a number of new astrometric, secular, and post-Keplerian measurements as part of updating the pulsars’ timing models. We also perform a thorough comparison of results between the narrowband and wideband data sets, validating the latter’s use for gravitational wave analyses. The results from searching the narrowband data set for a stochastic background of gravitational waves is presented elsewhere.