K.O.
I would check with your pediatrician on this one. To me it sounds like your daughter is too young for a booster. It would be one thing if she were a few weeks shy of 4 years old, but if she has more than a year to go, I think that's too early. I think there is some internal development that maybe occurs that makes it unsafe to use a booster before 4 years. But I don't know for sure. Why do you have to spend so much on a car seat for your son? There are other models that are just as safe and not quite so expensive.