Let's call the number we're trying to find "x". If 10% of x equals 4, we can write an equation to represent this:
0.1x = 4
To solve for x, we can divide both sides of the equation by 0.1:
x = 40
Now that we know the value of x, we can find 40% of it by multiplying x by 0.4:
0.4x = 0.4 * 40 = 16
So, 40% of the number is 16