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I just want to touch on a few things. It's possible that your seat is expired because Century products have not been made since 2004. Seats expire, usually around 6 years after date of manufacture. As the plastic ages, it becomes brittle. This could cause the seat not to function properly. It could even let the harness rip right out of the seat in an accident.
This is a (short, less than 10 seconds) video showing what happens in a crash test to an expired Britax convertible.
[ame="http://www.youtube.com/watch?v=wvCRz7BRAM0"]YouTube - Car seat expiration[/ame]
Also, Century seats maxed out the harness weight at 40 lbs, so you don't want to drive again until you have a new seat
Secondly, while your daughter is definitely at the minimums for a booster (4 years and 40 pounds), it's a rare 4 year old who can ride maturely in a booster. Until she's able to sit still 100% of the time and isn't at risk of leaning over, she needs a harness (most kids can do this by around the sixth birthday).
A harnessed seat like the Graco Nautilus or the Britax Frontier would allow you to keep her harnessed for a few more years and then move to a booster.
[ame="http://www.amazon.com/Britax-Frontier-Booster-Seat-Rushmore/dp/B001NPD72Y/ref=sr_1_1?ie=UTF8&s=baby-products&qid=1248797152&sr=8-1"]Amazon.com: Britax Frontier Booster Car Seat Rushmore: Baby[/ame]
[ame="http://www.amazon.com/Graco-Nautilus-Car-Seat-Matrix/dp/B0011URFRE/ref=sr_1_1?ie=UTF8&s=baby-products&qid=1248797186&sr=1-1"]Amazon.com: Graco Nautilus 3-in-1 Car Seat, Matrix: Baby[/ame]