My 20kg 4yo is in the maxi rider with the regular seatbelt and the anti submarining clip. We didn't get the harness as her eyes are already at the top of the seat.
But, that said I agree with Nics post. If I had known I'd just need the booster bit I would have bought just the booster that looks narrower and easier to use. But at the time we bought there weren't as many options availble.