If he is only 16 kilos at 4 years I would keep him in a seat with a five point harness....they are tested to 22 kilos and he is far safer in a five point harness than he ever will be in an h harness or seat belt booster. To get the maximum height wise I would suggest the baby love ezycombo or the maxi cosi complete air they Are the tallest of the harnessed booster seats. If you want to spend the money on another maxi rider do so but the ezycombo actually rates better in crash testing for what it's worth and is at least $200 cheAper.....baby bunting has it ozsale this week for $199 instead of $299 FTP. Just my five cents.
I would prefer a harnessed seat. What is weight limits of the ezycombo???? I thought with the boosters you could use a harness as well?? I had been told that once he hit 18kg he could not longer be in a 5 point harness is that wrong??