Chapter 9 - Analytic Geometry - Section 9.2 The Parabola - 9.2 Assess Your Understanding - Page 667: 2

To complete the square of $x^2−4x$, we add $4$.

In order to complete the square of $x^2+ax$, we must add half of the coefficient of $x$ and then take square of it. That is, $(\dfrac{a}{2})^2$ Since, we have: $a=-4$ Therefore, $(\dfrac{-4}{2})^2=4$ Thus, in order to complete the square of $x^2−4x$, we add $4$.