This article presents an advanced control strategy for power quality enhancement in standalone power supply systems (PSSs) with grid forming four-leg voltage source inverters (FL-VSIs). Indeed, an online adaptive reference generator (ARG) with a gray wolf optimizer (GWO) is proposed to sustain the control performances of a feedback linearization control (FLC) strategy and improve its robustness against load-side disturbances and system parameters' uncertainties. The key purpose of the proposed GWO-based ARG is to compensate for load and phase disturbances through smooth reference adjustments, in order to improve the voltage waveforms' quality and symmetry and conform to the existing power quality standards and metrics. The design methodology of the proposed control approach is thoroughly detailed, and its effectiveness is asserted through simulation and experimental tests, demonstrating its superiority in maintaining the voltage waveforms within the required standard limitations even under unbalanced and nonlinear loading conditions.