My 4 year old is in the Britax Regent and we like it. It isn't perfect, but I feel like he is safe in the 5 point harness. He loves his seat, he won't sit in anything else. I don't like that the straps twist (even though they say they don't) and that there is only 1 click when you buckle him into the crotch strap part, but you get used to it. I am looking for a seat for my 16 month old now that will last a while (he's too small for the Regent), so I have done a lot of research. I don't know of any where the backing removes later, but the following have a 5 point harness past 40 lbs (www.car-seat.org is a great site where you can get tons of info and reviews on all car seats. Many people have inquired about the >40 lb. 5 pt harness seats, so check the forums out for really good info):
Sunshine Kids Radian65 (5 pt to 65 lbs) and Radian 80 (5 pt to 80 lbs)-harness slots are the same height on both, so unless you have a heavier kid go with the Radian 65.
Britax Regent (5 pt to 80 lbs)- there is an update being made to this seat at the moment, so you may not be able to find it at the stores to look at. But, check out www.britax.com for more info.
Britax Boulevard, Marathon, and Decathalon (5 pt to 65 lbs)- As an FYI with these and all the other seats, your child may outgrow these height wise long before she outgrows weight wise. So, please check out the top harness slots. Many kids outgrow these before 4 years, but some last in them longer)
Apex 65
Evenflo Triumph Advance- just came out at Babies R Us and Walmart. 5 pt harness until 50 lbs., but the harness height is pretty high. Good value compared to the others, but you probably won't get to use it that long given that she is already 3.
Evenflo Titan Advance- just came out at Babies R Us and Walmart. 5 pt harness until 50 lbs., but the harness height is pretty high. Good value compared to the others, but you probably won't get to use it that long given that she is already 3.
Recaro Como & Signo- these are new car seats that are due out any time (como) & in August (Signo) (although these dates keep changing). They both have 5 pt harness to 70 lbs. Check them out on www.recaro.com (I am personally trying to hold out for these to be released as these look very promising)
So, it's a big purchase if you want a 5 pt. harness past 40 lbs. Not many of the seats are less than $300, but then again, you will be using the seat for a long time and your child's safety is worth it.
Good luck!