It's actually +6, since its +1 at 1, and then +1 every 4 (4, 8, 12, 16, 20), just to be picky. If you're looking for a bonus on the level of +1/2 levels, you might as well rewrite the CR calculation instead, since otherwise this makes it a no brainer race. With +6, it's already very close, or is.
Then again, given where this is going, rewriting the CR calc might be the best thing out there.