Often a good thing to think about when you are stuck is the information the problem gives you. We know basically:

1. We have a trapezoid $ABCD$ with base $AD$.
2. $AB=4$.
3. The trapezoid is inscribed in a circle, that is, $ABCD$ is a cyclic quadrilateral.

Keeping in mind all of this information should help get you started!

Let us know if you have more questions.