A variety of new ternary and quaternary metal–semiconductor inorganic nanostructures with unprecedented structural morphologies is achieved by the decoration of five monolayer-thick CdSe/CdS core/crown nanoplatelets with Au and Pt domains. Significant differences in metal growth behavior are observed by varying the CdSe core and the CdS crown dimensions. Depending on the core size, Au growth can be directed only to the CdS edges, or both at the edges and at the center of the nanoplatelets. In contrast, the nucleation of Pt domains always happens at the CdS edges independently of the core and crown dimensions. Furthermore, quaternary structures are obtained by additional Au growth on Pt-decorated CdSe/CdS nanoplatelets, where the effect of steric hindrance of the existing Pt domains results in the Au nucleation to occur only at the CdSe core. Instead, a change in the order of growth of the two noble metals results in Pt-Au alloys present only at the surrounding edges of the nanoplatelets. Additionally, the metal-decorated nanoplatelets are found to be efficient catalysts for H₂ fuel generation under white light irradiation. The highest apparent quantum efficiency measured is 19.3% ± 1.4% with a turnover frequency of ≈10⁵ molecules of H₂ per hour per nanoplatelet.