720 is correct. If the 1, 2, 3, and 10 are in the same position every time, and the other 6 balls are in random order, then here's the math.
6! (six factorial) = 6 X 5 X 4 X 3 X 2 X 1 = 720
I see that I'm repeating what someone else already said after I finished typing this, but here it is...