Thin films of the quaternary system FeCrMnNi were synthesised by ion beam sputter-deposition. The films were deposited on a silicon substrate at approximately 350 K. A high-entropy alloy thin film (HEATF) and a non-equiatomic thin film were obtained. Energy Dispersive X-ray Spectroscopy (EDX) and Transmission Electron Microscopy (TEM) were used to determine thin film composition and the atomic structure. The non-equiatomic thin film exhibited a polycrystalline structure with nanometre-sized grains. Microstructural analysis of the HEATF, which had close to equimolar composition, showed large crystals and planar defects. The microstructural differences between the HEATF and the non-equiatomic thin film are discussed in terms of current high-entropy alloy theory, previous work on thin films and nucleation theory.