At what age did you switch your child to a booster seat? Our daughter will be 4 in December & is 42 in and 40 lbs and looks really big in her car seat.
First I would suggest checking the manual for your dd's carseat and see what the weight limit is. Some go to 40 pounds, some go higher (some as high as 85 pounds!). If your dd has outgrown the weight limit (or height limit- the shoulder straps need to be at or above her shoulders) then she needs a new seat. I would suggest you look into higher harness seats that later turn into a booster (Graco Nautilus and Britax Frontier 85 are two examples). At not even 4 years old, I wouldn't be comfortable putting a child in a booster seat at all. My ds will be 8 in October and is still harnessed (in a Graco Nautilus, Britax Frontier 85 and a Britax Regent). He has a booster seat (Graco Turbobooster) that he has used on rare occasions, but his main seat is still harnessed.
I switched my oldest to a booster at 6.5 years, ~50 lbs. and about 46" tall. I switched my second last year, at 5 years, 45 lbs. and 44". We had carseats that harness to 65 lbs. My third child is only 1 and will be rear-facing for 2 or 3 more years, at which point I'll put her forward-facing until 5 or so.
If your daughter is over 40 lbs. she might need a new seat right away. If her shoulders are above the harness slots or she has outgrown the weight limit, her current seat is not safe. It is not safe to put a 3 yr. old in a booster. Many carseats are outgrown before a child is ready for a booster. There are lots of seats that will keep your daughter harnessed until she is old enough for a booster. The Evenflo Maestro is the most affordable, but the Graco Nautilus or Britax Frontier will last longer. They all convert to a booster when she is older.
In Wisconsin a child must be BOTH 1) at least 4 years old and 2) at least 40# to go unharnessed in a booster seat.
My dd turned 4 in February, but isn't 40#. I also don't really think she would always sit correctly in the booster without the harness. She would definitely want to reach things (including her rear-facing little brother) and would probably end up with an unsafe belt position. I'll probably just wait and consider it when she turns 5.