Some boosters are rated to 30 pounds. Check yours to see what it is in high back mode. The Turboboosters (is that what she's in?) is 30-100 pounds high back and 40-100 low back.
I think it's ok to have some seats rated to 30 pounds, but IMO they're for kids like my daughter and yours who are petite. Piper just today crested 30 pounds (only three more until she MUST forward face!! AHHHHHHH) and who is already four years three months. She's a safety nut and would do just fine in a booster, I'm sure. No way in heck am I putting her in one when she's still RFing in the first place, but I could go out today and get a Turbobooster and plunk her in it and it would be fine. Does she meet the minimum weight and age requirements that we recommend? Nope. But she won't be 40 pounds until she's six or so (I was six when I hit 40, and like your daughter will be), and by then she may be too tall in the torso for many convertibles. Certainly she would be for the 40# convertibles. So for her a booster rated less than 40 pounds is fine.
However, I do plan on keeping Piper in a harnessed seat until she's too tall, and then depending on how old she is I may move her from her Wizard to a Regent, rather than to a booster (though we'd likely get one Regent then a booster for my husband's car, and a travel booster).
Wendy