4 yrs, 40 lbs, and there's no point in having that particular seat if you're not going to use the harness to 65 lbs--that's the whole point in buying the seat.
It's just not as safe to be in a booster vs. a harness. Seat belts are only so effective. Harnesses work every time. At nearly four, she's old enough to be yelled at, given time outs, punished for unbuckling herself when the car is moving.
Don't switch her until she outgrows the harness, as the harness is safest. If you have the means already in place to keep her harnessed, then keep her harnessed.
She needs to be taught not to unbuckle herself--if she can unbuckle the harness, she can DEFINITELY unbuckle a seat belt much easier. Plus, there's a higher likelihood that she won't be sitting in it correctly and a higher likelihood that she'd be injured during a crash. Do yourself and her a favor and keep her in the harness and give her the ownership--let her buckle and unbuckle herself at the appropriate times so the novelty of being able to do it wears off and she does it when she's supposed to do it. If she unbuckles when you're driving, you pull over and refuse to drive until she is buckled.