How to simulate a control system with RK4

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My question is simple yet I could not find answer on the internet. When I have a system described by its differential equation, I can simulate its states from time steps to time steps with RK4. But what to do when I want to put an acting force into the system which I control. In RK4 I should know what value I am going to give into the system in the next time step. So far I tried to predict this value by the derivative of the input force. Can someone recommend me literature in this field? Or give me tips what to do in this case?