I just realized that our LO has been in size 4 diapers since he was like 10 or 11 months, so one and a half years! What?? I know 4 and 5 overlap somewhat, so we could switch to 5s, but we have no issues with fit or leaks, so we stick with 4s. He grew out of the other sizes so quickly. Has this happened with your kids? What size diaper did you stay in the longest? He's 28 months, 36" tall, and just shy of 32 lbs, so he's tall and on the skinny side.