Math:
http://www.abcya.com/fourth_grade_computers.htm
http://www.funbrain.com/FBSearch.php?Grade=4
http://www.math-play.com/4th-grade-math-games.html
http://www.mathplayground.com/grade_4_games.html
http://www.hoodamath.com/games/fourth-grade.html
Reading:
http://www.abcya.com/fourth_grade_computers.htm
http://www.funbrain.com/FBSearch.php?Grade=4
http://www.storylineonline.net/
Science:
http://mrnussbaum.com/fourth-grade-science/
http://www.uen.org/3-6interactives/science.shtml
Social Studies:
http://www.learninggamesforkids.com/social_studies_games.html
http://mrnussbaum.com/fourth-grade-social-studies/
Health:
http://www.healthyeating.org/Healthy-Kids/Kids-Games-Activities.aspx