Numerical and experimental studies of energy harvesting driven by vortex-induced vibration (VIV) are currently focused on arranging the energy-captured structure in a uniform incoming flow at a certain depth, ignoring the effect of the free surface on VIV. The fluid–structure coupling effect can be enhanced when a column-group structure with rigid connection is arranged under uniform flow, which is helpful for the structure to concentrate hydrokinetic energy from low-velocity water flow. In this paper, a staggered arrangement of a four-cylinder oscillator with rigid connections is proposed as the energy converter, and the fluid–solid interaction numerical method is carried out to simulate the VIV of the four-cylinder structure under single-phase flow and free surfaces. In U* = 2–16 (flow velocity U = 0.16–1.28 m/s), the results of the energy harvesting magnitude, efficiency, and density of the four-cylinder oscillator under the arrangement depth ratios S* = 2, S* = 3, S* = 4, and S* = 5 are compared with the results obtained in the single-phase flow. It was found that the column-group structure has a broader resonance range of VIV in single-phase flows than a single cylinder and can capture more hydrokinetic energy concentratedly from low-velocity flow. The VIV responses of the four-cylinder oscillator are suppressed at low submergence depths with a narrower resonance range, and its captured energy is reduced. In contrast, at high submergence depth ratio S*, the VIV responses are not suppressed obviously by the free surface. The magnitude of captured energy, energy-harvesting efficiency, and density of the four-cylinder structure are basically consistent with the results obtained in single-phase flow at S* = 5.