This book gathers the proceedings of the 4th International Conference on Water and Environmental Engineering (iCWEE), held in Sydney, Australia, on November 19-21, 2025. Contributions include a wide range of topics in environmental engineering such as flood hydrology and hydraulics, stochastic and urban hydrology, climate change impacts on water resources, drinking water treatment, computational fluid dynamics, fluid mechanics, resource recoveries from biosolids and wastes, circular economy in water and environment, emergent pollutants, innovations in wastewater treatment, interaction of surface and groundwater, water security, environmental sustainability, irrigation, and hydraulic engineering.