x^2 + 5x + 4 = 0
(x + 2.5)^2 - (2.5)^2 + 4 = 0
(x + 2.5)^2 - 6.25 + 4 = 0
(x + 2.5)^2 - 2.25 = 0 This is the answer if you complete the square.
However, if you wanted to find the solution to the equation, you would be better to factorise the equation:
x^2 + 5x + 4 = 0
(x + 4)(x + 1) = 0
x = -4 or x = -1