This paper has investigated the flow interference between four circular cylinders of equal diameters in a square arrangement in a cross-flow. Wind tunnel experiments were conducted to measure the mean force coefficients and Strouhal numbers (St) for six spacing ratios (l/d) varying from 1.5 to 4 at five subcritical Reynolds numbers (Re) ranging from 1.26×〖10〗^4 to 6.08×〖10〗^4. The pressure distributions on the surface of the cylinders were measured using pressure transducers. Furthermore, the hot-wire anemometer was employed to measure the vortex shedding frequencies behind each cylinder. It is revealed that for l/d=1.5, due to severe flow interference between cylinders, there is a difference between lift coefficients for the upstream cylinders. Also, for l/d≤2, the mean drag coefficients for downstream cylinders are negative. In addition, for l/d=2 and l/d=2.5, there is no vortex shedding from the upstream cylinders. Moreover, it is found that the effects of variations in this range of the Reynolds numbers, on the quality of the vortex shedding and Strouhal numbers are negligible; whereas the variations in l/d, strongly affect the flow pattern around the cylinders and aerodynamic coefficients. Also, by decreasing the amount of l/d, the effects of the flow interference between the cylinders increase.