Where are her shoulders in relation to the top harness slots of the roundabout? Her shoulders need to be AT or BELOW the harness slots, if she is above the slots it is outgrown and unsafe to use and you need a new seat now. She cannot ride in it. If her shoulders are at or below the harness lots she is safe to ride in it for now. The overall height limit is usually regarded as a guideline, and I would be comfortable making the parental decision to disregard it as long as her shoulders were below the top slots AND her ears are below the top of the shell.
That said, she is not old enough for a booster save an emergency situation. The bare minimum is 4 and 40 lbs. And even then, we like to see children harnessed until they are mature enough to sit correctly in a booster seat 100% of the time- usually between the 5th and 7th birthday. In fact, at her size, she would likely still fit rear facing which would be the safest way for her to travel (but forward facing is a completely acceptable option :thumbsup: )
Here are some forward facing options if she needs a new seat-
The Evenflo Chase ($60)- Top slots are around 17 inches, although it converts to a booster the fit is almost universally poor (she would need a different booster when the time comes to be boostered) , and as a 40 lb weight limit (which, at 34 lbs and almost 4 could take a 1-2 years to reach, but it doesn't really give you a good "cushion").
The Evenflo Maestro ($75)- Top slots are around 18 inches, it converts to a booster, and has a 50 lb weight limit. It is new, and I don't know how well it fits as a booster however it makes a very low booster and you would certainly need to purchase another booster down the road.
The Graco Nautilus ($150)- Top slots are around 18.5 inches, it converts to both a high back and backless booster, and has a 65 lb weight limit. The booster mode sometimes fits very well, sometimes does not- it depends on the child and vehicle. It is possible that you would not need to purchase another booster.
The Britax Frontier ($280)- The top are around 18.25 inches, it converts to a booster, and has a 80 lb weight limit. The booster mode usually provides a good fit and it is the tallest highback booster on the market. It is likely you would not need to purchase another booster.