1. Convert units:

* The thickness of the rock layer is 2 meters, which is equal to 2000 millimeters (2 * 1000 = 2000).

2. Divide the total thickness by the annual accumulation rate:

* 2000 millimeters / 4 millimeters/year = 500 years

Answer: It took approximately 500 years for the 2-meter layer of sedimentary rock to form.