Abstract— This article introduces a novel approach to enhance the performance of LCL based active power filters (APFs) in four-wire distribution systems by employing a nonlinear current control strategy. The strategy combines backstepping control (BSC) and a nonlinear disturbance observer (NDOB) to effectively manage harmonic and interharmonic grid currents. By situating the shunt active power filter (SAPF) at the point of common coupling (PCC) via the LCL filter, the technique ensures that grid connected currents remain balanced and purely sinusoidal. The integration of NDOB with BSC aims to fortify the resilience of BSC against disturbances. Consequently, any disturbances occurring within the system are precisely estimated by the NDOB and subsequently mitigated through the BSC mechanism. Notably, this approach showcases robust adaptability across diverse scenarios, encompassing external disturbances, variations in filter parameters, nonlinear loads laden with harmonics and interharmonics, load imbalances, and non-ideal grid voltages. Its performance remains robust and stable even when disturbances are present. Comparative analysis with linear-based methodologies underscores the advantages of this approach, revealing quicker and smoother transient responses. The efficacy of the proposed technique is demonstrated through comprehensive simulation studies, substantiating its potential for significantly advancing power quality in complex distribution systems.