A group of students is arranging squares into layers to create a project. The first layer has 4 squares. The second layer has 8 squares. Which formula represents an arithmetic explicit formula to determine the number of squares in each layer?

a1 = 4; an = 4 ⋅ a(n − 1), n > 0
a1 = 4; an = 4 ⋅ a(n + 1), n > 0
a1 = 4; an = 4 + a(n + 1), n > 0
a1 = 4; an = 4 + a(n − 1), n > 0