At 43" tall, there are at least 5 seats I can think of she would easily fit into. My 4.5yr old is 43" tall and can easily fit into many seats.
Graco Nautilus ($150) will harness to 65lbs, has a 17" top harness slot, and fits most kids to age 7 in the harness. It then becomes a good booster to 100lbs. This is a great seat, and it comes in PINK! :)
Britax Frontier ($280) will harness to 80lbs, becomes a good booster, has a 16.5" top harness slot, and fits most kids to age 6 in the harness. Also comes in pink. Can be difficult to install and its large.
Evenflo Generations 65 ($99 at Babies R Us), will harness to 65lbs, has a top harness slot of 17", and fits most kids in teh harness to age 6/7, then becomes a booster seat. This is a great seat for the money, I really like it. My friend just purchased two of them for her 4yr old and 2yr old.
A child shoudl remain in a 5pt harness until they are at least 4yrs and 40lbs. These are BARE MINIMUMS. A child must be able to sit properly in the seat belt, upright, not leaning or sleeping, leave he belt alone, at ALL TIMES. Sometimes that maturity is too much unti la child is 5-7yrs old. she is for sure safer in a harness, and the ones I listed above would work beautifully for her for at least 2 more years in the harness. My 4.5yr old is 43" tall and 40lbs, and fits in all the seats above with tons of room to grow. he too is still in a 5pt harness and will be for at least 2 more years.
A car seat is outgrown forward facing when one of three things happens, the child hits the forward facing weight limits, the ears become level with the seat back, or the shoulders go over the top harness slot. A child should remain in a 5pt harness as long as possible, in a higher harness weight car seat.
She is safer in the harness mama, for what you would spend for a good booster right now, spend a few extra dollars and get her a harness car seat to keep her safer for a few more years ,and then move to a booster seat.