Not all of the offspring will have it. That is why the parents have 5 toes. Somewhere in one of their backgrounds, there is a 4 toed bird. If you are breeding, I would not breed the bird with 4 toes and actually would try and figure out if it is the roo or the hen that is throwing the 4-toers and get a different bird.
Bred to a 5 or 6 toes bird and the offspring should have the correct number of toes. That said, you always want to breed birds with the best qualities and not breed birds with lesser qualities. Only you can decide if the 4-toed bird is worth breeding or is better passed elsewhere.