Savage Wombat
Hero
Really, losing half your hit dice and the associated BAB/skills/saves is not much? On the occasion that your invisibility and spell-like abilities don't work, you're pretty helpless. I'd say they better be nice.
I interpreted his statement to mean he'd be playing a pixie / rogue 4 among the eighth level party, so he'd not be losing HD at all. If you interpret him to mean he'd be playing a pixie with 4 HD and ECL+4, your statement would be accurate. Pathfinder doesn't use ECL, so I didn't want to assume.
But nothing wrong with ratfolk.