Tetrametallic core–shell Ag@CoNiFe nanoparticles (NPs) supported on graphene have been synthesized by a facile one-step in situ procedure for the first time. We tuned the reducing ability of the reductants to control the structure and particle size of the as-synthesized NPs. The use of weaker reducing agents, such as ammonia borane (AB) and methylamine borane (MeAB), result in the core–shell structure, whereas the stronger reducing agent NaBH4 results in alloy NPs. The as-synthesized core–shell NPs reduced by MeAB possess a smaller particle size and exhibit a superior catalytic activity compared to the core–shell NPs reduced by AB and alloy NPs reduced by NaBH4 for the hydrolytic dehydrogenation of AB. Moreover, the turnover frequency of the as-synthesized tetrametallic core–shell NPs is 118.5 mol min−1 molAg−1, which is higher than that of Ag-based trimetallic, bimetallic core–shell, and alloy counterparts and even higher than that of most reported noble-metal-based NPs. Kinetic studies indicate that the catalytic hydrolysis of AB and MeAB by the Ag@CoNiFe/graphene NPs is a first-order reaction. Furthermore, the as-prepared NPs exert good catalytic activities and recycle stabilities towards the hydrolysis of AB. Additionally, the as-prepared NPs display magnetic recyclability, which makes the practical recycling of the NPs more convenient.