K.M.
Legally, I think she can transition to a booster at 4years old AND 40lbs. However, I would keep her in a 5-point harness as long as possible. My oldest is 5 1/2 (he's on the smaller side for his age) and he's still in a 5pt harness. We bought the Britax Frontier. It's good for up to 80 or 85lbs.