Fine quaternary single-domain Heusler alloy nanoparticles with a composition of Fe2CoGa0.5Al0.5 have been synthesized using a simple template-less chemical route for the first time. Ab initio calculations and standard models have been used to analyze and interpret the experimental findings. The synthesized nanoparticles with an average crystallite size of 42 nm exhibited high saturation magnetization (>5 μB f.u.-1), high effective anisotropy constant (≈8 × 106 erg cc-1), high Curie temperature (>1200 K), low magnetic remanence (<0.2 μB f.u.-1) and low coercive field (<90 Oe), declaring their suitability for fabricating various nanomagnetic devices.