1. Convert everything to the same units:

* Since the sediment accumulation rate is in millimeters (mm), convert the rock thickness to millimeters: 2 meters * 1000 millimeters/meter = 2000 millimeters

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

* 2000 millimeters / 4 millimeters/year = 500 years

Therefore, it took approximately 500 years for the 2-meter thick sedimentary rock layer to form.