Anionic exchange membrane (AEM) water electrolyzers are emerging as a cost-effective technology for green hydrogen production. However, state-of-the-art AEM electrolyzers rely on platinum group metal (PGM) catalysts for the hydrogen evolution reaction (HER). Currently, PGM-free HER catalysts exhibit inadequate activity and stability at high current densities in electrolyzer environments. Here, we report a simple electrodeposition method for a self-supported Ni4Mo-MoOx catalyst. This catalyst exhibits remarkable HER activity, as demonstrated both in three-electrode cells as well as in prototype AEM electrolyzers. In particular, the catalyst enables AEM electrolyzers to operate stably at current densities as high as 3 A cm-2, which had not been reported for a non-PGM HER catalyst. The performance (2 V@3 A cm-2) is comparable to the benchmark Pt/C, whereas the stability is even higher. Characterization and particularly operando X-ray diffraction and absorption spectroscopy reveal that the catalyst is an unconventional tetragonal Ni4Mo with a D1a superlattice whose surface contains in situ formed MoOx species. The cooperative action of MoOx and Ni4Mo enhances the volmer step of HER, attributing to the superior activity.