Answer to Question #44762 in Trigonometry for Stephanie

Question

Model equation for tide: h=2 cos⁡(π/6 t-2π/3)+4
Given the above, a large boat needs at least 4 meters of water to secure it at the end of the pier. Determine what span of time after noon, including both a starting and ending time, the boat can first safely be secured, justifying your answer.
So far, I have gotten this, but I'm stuck!
4=2 cos⁡(π/6 t-2π/3)+4
0=2 cos⁡(π/6 t-2π/3)
0=cos⁡(π/6 t-2π/3)
0=cos⁡(π/6 t) cos⁡(2π/3)+sin⁡(π/6 t) sin⁡(2π/3)

Am I even remotely on the right track??? Can someone PLEASE help?!?