Quad roots of 1???

+1 -1 +i -i are the roots.

$a,b,c,d$ are the roots of $x^4-1=0$ , the sum of the roots are minus the coefficient of $x^3$ , so the result is : $0$ .

Note that this is the same as finding the solutions to $x^{4}=1$ . The solutions to this are the fourth roots of unity. Whenever you sum the values of the $n^{th}$ roots of unity you will get the answer zero.