These are good sites for teachers to access. They may be too sophisticated for fourth graders to access on their own. However, they contain a wealth of information.
This is the official home of the NASA education Division's electronic publications and NASA Television's educational schedule. You can search all of NASA from this site.
This is the table of contents page for instructional materials. Teachers should preview the site to check appropriateness for your grade level.
This is a list of Internet links for elementary teachers. It includes science sites as well as other subjects areas. A great starting point for exploration.