Both the first and third terms are perfect squares.
$$ x^2 = \left( \color{blue}{ y } \right)^2 ~~ \text{and} ~~ 4 = \left( \color{red}{ 2 } \right)^2 $$The middle term ( $ 4x $ ) is two times the product of the terms that are squared.
$$ 4x = 2 \cdot \color{blue}{y} \cdot \color{red}{2} $$We can conclude that the polynomial $ y^{2}+4y+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 = y } $ and $ \color{red}{ B = 2 } $ so,
$$ y^{2}+4y+4 = ( \color{blue}{ y } + \color{red}{ 2 } )^2 $$