You use the quadratic formula.
x = −b ± √(b^2 − 4ac)/2a where y = ax^2 + bx + c
b is 0, a is 1, and c is 4.
-0 ± √(0^2 − 4 x 1 x 4)/ 2(1)
±√(-16)/2
Split √(-16) into √(16) and √(-1).
Now, i = √-1
So you get ±√(-16) = ±4i.
The original equation was ±√(-16)/2.
±4i / 2 = ±2i
The answer is x = ±2i
(Note: ± means positive or negative)