The structure and composition of Fe films grown on As-terminated GaAs(001)-2x4 surfaces at 175 degrees C has been studied in situ with scanning tunneling microscopy (STM), photoelectron diffraction (FED), and x-ray photoelectron spectroscopy (XPS). The GaAs surfaces were prepared by molecular beam epitaxy (MBE) and exhibited large atomically well-ordered terraces. We find that the 2x4 reconstruction has a significant impact on the Fe nucleation and growth, with initial nucleation occurring at As-dimer sites. STM reveals that the first half-monolayer of Fe forms small two-dimensional islands along the As-dimer rows before growing onto the adjacent Ga-rich rows, with no evidence of substrate disruption. FED indicates that the growth is predominantly layer by layer, with the growth front for the nth deposited layer limited to the (n+1)th layer. XPS spectra show that the Fe films include a concentration gradient of Ga and As out-diffused from the interface, with some of the As segregating to the Fe surface, similar to previous results obtained for growth on non-MBE prepared GaAs surfaces. Possible mechanisms for the film growth and the origins of the intermixing are discussed.