Both the first and third terms are perfect squares.
$$ x^2 = \left( \color{blue}{ w } \right)^2 ~~ \text{and} ~~ 4 = \left( \color{red}{ 2 } \right)^2 $$The middle term ( $ -4x $ ) is two times the product of the terms that are squared.
$$ -4x = - 2 \cdot \color{blue}{w} \cdot \color{red}{2} $$We can conclude that the polynomial $ w^{2}-4w+4 $ is a perfect square trinomial, so we will use the formula below.
$$ A^2 - 2AB + B^2 = (A - B)^2 $$In this example we have $ \color{blue}{ A = w } $ and $ \color{red}{ B = 2 } $ so,
$$ w^{2}-4w+4 = ( \color{blue}{ w } - \color{red}{ 2 } )^2 $$