In the picture she looks like she's bending, so I don't think I can get a good size estimate from the picture. She looks fine.
She may be a little small yet, since I assume you let them start raising their own queen 4 weeks ago, she's just getting going. I find that queens not laying full blast are smaller (and she wouldn't be up to her full potentional if it was a 3 or 4 frame split). Emergency queens can be smaller too, but I judge a queen by her laying and pattern, not by size.
Rick