Thank you, clarkjd, for your thoughtful response.
Dogman_1234, I'm sorry to inform you that the answer is not 2.
What must be considered here is the range of the square root operation. I did not understand that the square root symbol implies that the radical is to be solved only for the principal square root. This means that the range of the radical, when solved, is [0, ∞). No input value, x, will produce an output of -4. Therefore, x=4 is not a solution.