When can my child use a high back booster?

My daughter uses a Graco Nautilus 3 in 1 car seat and has just now figured out that she can unbuckle it by herself. We have only had one episode when she did it while the car was moving, but I'm wondering if I should switch her to using the high back booster with the regular sholder strap seat belt. The... show more My daughter uses a Graco Nautilus 3 in 1 car seat and has just now figured out that she can unbuckle it by herself. We have only had one episode when she did it while the car was moving, but I'm wondering if I should switch her to using the high back booster with the regular sholder strap seat belt.
The sticker on the side of the seat says that the child must be at least 3yr old, 30lbs, and 38" tall. My daughter is almost 4, 34.6lbs and 39" tall.
Technically, she is big enough, but is it safe when she is this young?
I'm just curious about when everyone else generally puts their child in a high back booster and what are some of the pros and cons.
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