Online games allow students to assess their learning in a private way. They see how much they know, but they do not have to worry about being graded by teachers or judged by their peers. Have them visit Just Kids Games. They can play "It's Common Sense," where they will have to figure out which sense is being used in different scenarios, or "Ralphie Recycles His Room," to learn about different items that can be recycled.
Engaging in Web Quests gives students background information about various issues, as well as teaches them how to solve relevant problems by themselves. Have them visit Cloud Quest, where they will learn about different types of clouds on their journeys to become meteorologists. Fourth-graders also can see some Web Quests on plants, to discover the various parts of plants. Have them work in teams to solve the problems and build interpersonal skills, or allow them to work individually on such tasks.
At the fourth-grade level, students have most likely not engaged in too many research projects before. Teach them how to use the Internet for research, how to limit searches to websites with .edu or .gov endings and how to cite their sources in their papers. Assign them a grade-appropriate topic such as parts of an animal cell, the life cycle of the monarch butterfly, the phases of the moon or weather instruments.
If you do not have the time, space or resources to allow students to do actual labs in the classroom, have them watch some labs online. Available topics are wide and varied and include animal behavior, virtual DNA labs, solar system simulators, graphing labs and so forth. After the class has watched the lab online, give the fourth-graders a series of questions to respond to, ensuring they have understood what they just watched.