Let a,b,c,d be the roots of P(z)
Consider sum of each of the square of the roots i.e
a^2 + b^2+ c^2 + d^2 = (a+b+c+d)^2 - 2($sum of roots 2 at a time$)
You'll see that this is negative, that proves that there is at least one complex root, since P(z) has real coefficients, the 2nd...