I am so confused about booster seat, carseat, high-back, no back.... My son is a big four year old, 44 inches, 42 pounds, and just turned 4. I want the very safest carseat, booster seat, etc but I do not know which choice(no back booster, high back booster, combo carseat) is the absolutely safest because of all the different types and I assume the safest choice changes as the child gets older. Price is no concern as I am not rich, but this is just too important to be cheap. He is currently in a combination carseat, which was suppose to last until he is 80 pounds, but it just looks too small.