The depth of water in a pond is modeled by a differentiable function D(t) (in meters) for 0≤t≤10, where t is measured in hours. The derivative satisfies D′(t)>0 for 0<t<4, D′(4)=0, and D′(t)<0 for 4<t<10. Explain what happens to the water level at t=4 hours and classify this time as a local maximum, a local minimum, or neither.