Dear Rahul i didn't exactly understand the meaning of 'lowest polynomial'. However, i consider it as a polynomial of lowest degree and solve the problem as below:
(x2+x+1)(x2-x+1) = x4 + x2+ 1
Thus, if we add x2 to above term it becomes a perfect square i.e.
(x4 + x2+ 1) + x2 = x4 + 2x2+ 1 = (x2+ 1)2