The roots of x4 – x3 – x2 – 1 = 0 are a, b, c, d. The numerical value of f(a) + f(b) + f(c) + f(d), where f(x) = x6 – x5 – x3 – x2 – x is

1) -4
2) 2
3) 6
4) None of these