I want to add, with regard to the moral dillemma, that it is legal to put a 4yo in a booster - and depending on the child/situation, some would argue safe. Older/bigger 4 for short rides in a pinch? Emergency back up plan? 2nd car that is hardly hardly ever used? Taxi ride? Now my *personal* ideal is to keep a child harnessed in the full time vehicle until at least 5 1/2 for various reasons, comfort, ease of self buckling, etc, but if someone were to use a booster for a 4yo on a short trip, I would not automatically judge and say it was unsafe - you just don't know their situation. So given that people DO put 4yos in boosters, even if you personally do not agree with it, giving an assessment on this given seat's merits and shortcomings as well as how well it fits can only improve safety in those situations. Suppose this seat does NOT fit your child at all, someone with a more averaged sized 4yo can know to rule it out. Suppose it fits like it was made for him, a parent with a similarly sized child can use that information. Your child will not be in any danger in this situation, since you won't even have to leave your driveway to assess, and your input can only help other parents. By all means state your reasons for not boostering a 4yo, as this information may be helpful to other parents, but a straight forward assessment of what is good and not good about this seat does not necc. need to contradict your own personal beliefs.
You could even help a parent with an older child who is on the small side, which is the situation I am likely to find myself in, as one of my girls is much smaller than the other, but I plan to booster them next year.
I feel like you are afraid that writing this review will compromise the safety of other children, but I think your review can only be beneficial.