Car seat safety ??

So my just turned 4 year old (a few months ago) is still rear facing. Her car seat maxes out rear facing at 40 lbs. she typically weighs 37-38 lbs. the other night she stepped on the scale and it was 39 lbs. I assume this is just after dinner etc (she likes to step on the scale whenever she’s in my bathroom to look at the numbers and have me tell her she’s growing so good lol). But I’m wondering about when to switch her car seat. I assume I wait to turn her until she’s consistently 40 lbs? Or does it become unsafe getting close to the 40 lbs? I just want to do whatever is the safest for her. I guess that part has just never been clear to me