Very interesting question!
But, √-4 is not a real number; in fact, the result of √-4 is 2i.
The i is an imaginary number and it basically is a solution for √-1
(So, i^2 is = -1).
Since -4 is negative, the square root doesn't exist (in terms of real numbers) just because you can't multiply any two identical numbers to get a negative result.
-2 x -2 = +4
2 x 2 = +4
That is why √-4 is NOT considered a real number.
Hope I could help you out!
Have a good one, and God bless!