In this article we present a flatness-based controller that solves the trajectory tracking problem for an unmanned four-rotor aircraft. A simple version of the dynamical system obtained from a suitable coordinate transformation of the Euler-Lagrange model of the system is used. This model, combined with a suitable input saturation nonlinearity, also avoids the inconvenient system singularities. The corresponding tracking error is shown to be globally asymptotically stable but only locally exponentially stable, while the controller maintains the four-rotor system motions inside a feasible region. Numerical simulations are presented to assesses the effectiveness of the proposed control strategy.