The charge on a capacitor plate in a circuit, as a function of time, is shown in the figure: What is the…

As we know that the $\frac{dq}{dt}$ at any point t gives as current at that point

and in this case the curve is constant  at the point of 4 so that $\frac{dq}{dt}$ is zero

Since ${\left.\frac{dq}{dt}\right|}_{t=4s}=0$

Therefore current $=0$