I said 3-4. DD moved to a booster age 3, 110cm, not sure on the weight then. I was a single parent, she rode in a car maybe 1-2 times a month, we had a FF silvercross stage 1-2 seat, but she was so tall that the highest insertion for the shoulder straps was below her shoulders (she actually grew out of the seat earlier but the plastic base and foam rubber they use had an extra set of insertion slots which the over-slip cover didn't, so i cut the cover to utilise them for another few months). I simply couldn't afford one of the harness seats which fits taller-than average kids.
After we moved in with DH she now rides in the car 4x a week, and we used the seat with the belt for a time but she quickly outgrew the back (it's adjustable but we have non-removable headrests in our back seats which prevent her headrest adjusting up much and affect the stability of the seat if we raise it anyway). We then moved her to a booster cushion, which i replaced just yesterday with a high-back booster which i'm far happier with (we have a mazda rx8 which doesn't take many conventional seats and no money to spend on really expensive seats - i found a high back with adjustable rake which happens to fit in our back seat on offer yesterday and snapped it up!). I intend to keep her in the high-back for another few years.
For the new baby we have a RF 0 infant seat good to 8kg, and after that we were hoping to buy a RF good to age 4 or so, but there are none available in the UK which will fit into our car. At the moment we cannot afford to replace the car, so i guess time will tell on that. Baby isn't due until June so we have hopefully 15months to worry about it before making a decision.