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(eng): Underactuated mechanical systems are systems with less actuators than degrees of freedom. Therefore, it is complicated to measure all states, i.e. angular positions or angular velocities, of the mechanical system. Alternative solution consists in an observer design such that unmeasurable states are estimated. For this purpose, a high gain observer for an Acrobot was introduced. By virtue of an Acrobot embedding into a 4-link model, the high gain observer was simply extended and applied to the 4-link model. The main aim of this paper consists in a coupling of a method of the Acrobot embedding into the 4-link model and the high gain observer design for the Acrobot. The coupling results in an observer for the 4-link model with the same structure as the already developed high gain observer for the Acrobot model.