A nonlinear optimal control approach is developed for the quadruple water tank system. Approximate linearization around a temporary operating point is applied to the nonlinear dynamic model of the four water tanks which are fed by two pumps. The linearization makes use of Taylor series expansion and of the calculation of the system's Jacobian matrices. For the approximately linearized model of the four water-tank system an H-infinity feedback controller is designed. This controller stands for the solution of the nonlinear optimal control problem for the system under model uncertainty and external perturbations. For the computation of the controller's feedback gain an algebraic Riccati equation is iteratively solved at each time-step the control method. The global asymptotic stability properties of the control method are proven through Lyapunov analysis.