We present the fourth Open Gravitational-wave Catalog (4-OGC) of binary neutron star (BNS), binary black hole (BBH), and neutron star-black hole (NSBH) mergers. The catalog includes observations from 2015 to 2020 covering the first through third observing runs (O1, O2, O3a, and O3b) of Advanced LIGO and Advanced Virgo. The updated catalog includes seven BBH mergers that were not previously reported with high significance during O3b for a total of 94 observations: 90 BBHs, 2 NSBHs, and 2 BNSs. The most confident new detection, GW200318_191337, has component masses 49.1 − 12.0 + 16.4 M ⊙ and 31.6 − 11.6 + 12.0 M ⊙ ; its redshift of 0.84 − 0.35 + 0.4 (90% credible interval) may make it the most distant merger so far. We estimate the merger rate of BBH sources, assuming a power-law mass distribution containing an additive Gaussian peak, to be 16.5 − 6.2 + 10.4 ( 25.0 − 8.0 + 12.6 ) Gpc−3 yr−1 at a redshift of z = 0 (0.2). For BNS and NSBH sources, we estimate a merger rate of 200 − 148 + 309 Gpc−3 yr−1 and 19 − 14 + 30 Gpc−3 yr−1, respectively, assuming the known sources are representative of the total population. We provide reference parameter estimates for each of these sources using an up-to-date model accounting for instrumental calibration uncertainty. The corresponding data release also includes our full set of subthreshold candidates.