Well because it's fourth graders, your going to be looking for recent pop music, you might want to check out trl's top ten, or the billboard charts, Ill recommend a few though( just want to note that I don't listen to popular music, so if I made a mistake, or someone doesn't agree, I won't care because I listen to metal

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1. Sos-Rihanna
2. Unwritten-Natasha Bedingfield
I wish I could give you more, but all I am doing is just looking at the billboard charts, I am really out of touch with what fourth graders listen to, i'm a junior in high school, but if your a teacher my best advice to find out what they listen to is just ask them, they're pretty honest at that age, if you need more help just post again. peace.