Abstract: Flow-induced vibrations (FIVs) of four square-arranged circular cylinders have been investigated in a wind tunnel at Re = 3200 ~ 28900. The spacing ratios S/D are from 1.5 to 4.0 (S is the center-to-center spacing between two cylinders and D is the cylinder diameter) and the mass ratio m^* ≈ 345.0. Results show that, when S/D ≤ 2.0, cylinder 1 and cylinder 2 (upstream cylinder) present an combined vortex-induced vibration (VIV) and galloping-like response, while cylinder 3 and cylinder 4 (downstream cylinder) present an combined vortex resonance (VR) and wake-induced galloping (WIG) response. When S/D = 2.5 ~ 3.0, cylinder 1 and cylinder 2 are basically close to the VIV of a single cylinder except for weak vibration for large reduced velocity. Both of cylinder 3 and cylinder 4 present a separated VR and WIG response with different amplitudes and response regions. At S/D = 4.0, cylinder 1 and cylinder 2 are almost similar with the single cylinder, while cylinder 3 and cylinder 4 still present a separated VR and WIG and the vibration responses between the downstream cylinders are very close. When the cylinder exhibits VIV (upstream cylinder) or VR (downstream cylinder), the vortex shedding frequency f_s will significantly be locked to the first harmonic of the natural frequency f_n. As the reduced velocity gradually increases, the cylinders present intense galloping-like response (upstream cylinder) or WIG (downstream cylinder) and the shedding frequencies f_s show distinct multiple harmonics. While, the vibration frequency f_o only can be locked to the natural frequency f_n. By summarizing the vibration amplitudes characteristics of the four square-arranged and the two cylinders, it can be found that there exist similar vibration patterns among them and the dynamic response law of the two cylinders system are still applicable to the four square-arranged cylinders.