M.H.
I haven't read ALL the other posts...but I just recently bought my four year old a booster seat. It is a simple Graco from Walmart.
I recommend the kind with a BACK (which has an option of becoming back-less) for a couple of reasons. FIRST, from everything I have read, you child needs a back if the car you have doesn't have a headrest AND if your child is under 40 pounds. SECOND, I love that our new booster seat headrest comes out and allows my son to rest his head on the side of the headrest so he can sleep more comfortably without his neck getting all kinked. I'm so worried about his comfort in the car and I have to say, we love the new boosterseat.
Good luck with your decision if you haven't made one already!