This study investigates the microstructure after quenching and the precipitation behavior during subsequent tempering in Fe-2.5Ni-0.5Al, Fe-2.5Cu, and Fe-2.5Cu-2.5Ni-0.5Al (wt%) steels, with and without Mo additions. All alloys were solution-treated at 900 °C for 60 min, followed by quenching and tempering at 550 °C for up to 40 min. Microstructure and precipitation characteristics were analyzed using microscopy, atom probe tomography, and in situ small-angle X-ray scattering, supported by thermodynamic calculations and continuous cooling transformation diagram simulations. The Fe-Ni-Al steels (with or without Mo) exhibited a ferritic–bainitic microstructure. The Fe-Cu steel was primarily ferritic, while Mo addition promoted a ferritic-bainitic structure. The Fe-Cu-Ni-Al steel displayed a ferritic–martensitic microstructure, which transformed into a fully martensitic structure with Mo addition. During tempering, no precipitates were detected in the Fe-2.5Ni-0.5Al steel, whereas Cu-rich precipitates formed in both Fe-2.5Cu and Fe-2.5Cu-2.5Ni-0.5Al steels. The enhanced bainitic/martensitic transformation induced by Mo addition resulted in a higher dislocation density after quenching, which facilitated Cu precipitate nucleation during tempering. Hybrid Monte Carlo/Molecular Dynamics simulation confirm that Mo alters the matrix distortion in Fe-2.5Cu-2.5Ni-0.5Al steel, a key factor influencing nucleation and precipitation kinetics. Moreover, the addition of Mo reduced precipitate growth and coarsening, contributing to the retention of high hardness after tempering.