Car seat laws are a minimum standard. They aren't necessarily what is safest, but rather what is "allowed by law". It's a personal decision to be sure, but I'm proud to say that my 4 year old, 40 pound child is still harnessed in a Graco Nautilus and will be for quite a while.
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Another person here who LOVES the Nautilus! DD is 3.9 yo/39 pounds. She still fit in her Marathon, but one more growth spurt and her shoulders would be too high for the harness slots, plus since it has a thick base on the bottom (recline pivot for rear-facing infants), she sat so high and had little leg room in the car. I scored a Nautilus on clearance at Target for 50% off since they changed them from grey to brown!

Now in the Nautilus she can still take naps w/o falling forward, she loves the cup holder and she sits much lower giving her normal legroom... plus she is still in a 5 point harness! For travel we gate-check the Cosco high-back booster (can 5 pt to 40 pounds) and as a spare/emergency seat we have the Graco Highback booster. She does well in the highback, but honestly I feel she is so much safer in the Nautilus. In time when I feel she is ready for a backless booster both Graco seats we own will convert to backless so we don't need to buy anymore seats!