The rule on how Unnatural Characteristics work has changed in the Black Crusade Core Rulebook, and makes it a much better utility to both players and GM. Rather than being a multiplier that can just get insanely out of hand, it comes in ranks, gaining additional degrees of success equal to half the ranks rounding up. The ranks start at 1 and continue on until forever. Instead of multiplying your Characteristic bonus, they add onto your Characteristic bonus. So with a Strength of 30 and Unnatural Strength (+ 4), your total Strength Bonus comes out to 7 and gains an additional 2 degrees of success on Strength checks.
In this case, certain numbers should be changed to reflect this. Orks have Unnatural Toughness (+ 3), Kroot have Unnatural Strength (+ 3), Eldar have Unnatural Agility (+ 3), Space Marines have Unnatural Strength (+ 3) and Unnatural Toughness (+ 3), and the Cerebral Implants grant Unnatural Intelligence (+ 2) and a + 20 bonus to Logic and Lore tests. The ranks of Unnatural Statistics gained through a Race or Implants are not counted towards the XP required to gain Unnatural Statistics through a Career’s progression table.