National Geographic for Kids http://www.nationalgeographic.com/kids/
AAA Math for 4th Grade http://www.aaamath.com/grade4.html
Interactive Math http://nlvm.usu.edu/en/nav/grade_g_2.html
Cool Math http://www.coolmath.com/
Math Cats http://www.mathcats.com/
Young Investors http://www.younginvestor.com/
Funbrain - Numbers http://www.funbrain.com/numbers.html
List of Many Other Good Sites http://www.geocities.com/EnchantedForest/Tower/1217/math.html