Here are some internet links that 4th grade students enjoy:

www.funbrain.com This is a fun site where you can play learning games that go along with our fourth grade curriculum.

http://www.internet4classrooms.com/skills_4th_original.htm  Fourth grade skill builder site. 
Explore games in language arts and math!

http://www.sheppardsoftware.com A website filled with fun learning games!