First, since there are two numbers, i and -i, in the complex numbers, whose square is -1, it is better to write your first line as "x= i".
Your error is where you go from "\(\displaystyle x^4= 1\)" to "\(\displaystyle x= 1\)". \(\displaystyle x^4= 1\) is a fourth degree polynomial so has four solutions in the complex numbers, 1, -1, i, and -i.