Questions: Complete the square. Fill in the number that makes the polynomial a perfect-square quadratic. j^2 + 4j + □

Complete the square for the quadratic expression $j^{2} + 4j + \square$.

Identify the coefficient of the linear term.

The coefficient of $j$ is $4$.

Calculate half of the coefficient of the linear term.

Half of $4$ is $2$.

Square the result to find the constant term.

Squaring $2$ gives $4$.

The number that makes the polynomial a perfect-square quadratic is $\boxed{4}$.

The number that completes the square is $\boxed{4}$.