∠∠A = 30

∴CD= 1/2BC= 1/2AC=2

∴s= 1/2×ab×cd= 1/2×4×2=4

If you are on the left, you will use the trigonometric function of 75, so it is more troublesome, but the method is the same, such as the left picture, so you can calculate.

I hope I can help you, O(∩_∩)O Thank you ~