Professor Smart’s Safe

Professor Smart has locked his lunch in his safe and forgotten the nine digit combination.

However he does recall that the digits in the combination (abcdefghi) conform to these rules:

d - f = g

a * b * c < h * i

b + c + f < h

a = b + i

a = 4

h > 6

i < h

Can you design a decision model that will help Professor Smart get his lunch?