Hi there!
The general formula used to find the area of a circle is :
A = [tex]\pi[/tex] × r²
Since you know the diameter of the circle, you need to divide it by two in order to know what your radius (r) is :
4 ÷ 2 = 2
Now that you know that your radius (r) is 2, replace it in the formula and solve the equation :
A = [tex]\pi[/tex] × 2²
A = [tex]\pi[/tex] × 4
A ≅12.57
There you go! I really hope this helped, if there's anything just let me know! :)