My DS is smaller than your DD and he's in a 5. I think it's time to go up. Plus I notice that Huggies are narrower. I like Pampers and Luvs. They are a bit wider in the booty area.
I would go up to size 5 and perhaps try a different brand. This post reminds me of the SNL "commercial" for Huggies (or was it Pampers?) Thong Diapers. It was pretty funny!
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Re: Diapers-G String