This paper presents a practical method for small-scale VTOL design. It helps for elements selection and dimensioning.We apply the latter to design a fully autonomous quadrotor with numerous innovations in design methodology, steering and propulsion achieving 100% thrust margin for 30min autonomy. The robot is capable of rotational and translational motion estimation. Finally, we derive a nonlinear dynamics simulation model, perform a simulation with a PD test controller and test successfully the robot in a real flight. We are confident that "OS4" is a significant progress towards intelligent miniature quadrotors.