These websites have tons of science experiment ideas appropriate for 4th grade students. Some look like a lot of FUN!
http://www.mpmschoolsupplies.com/ideas/science-fair-projects/grade-4/
https://www.education.com/science-fair/fourth-grade/
http://www.mpmschoolsupplies.com/ideas/science-fair-projects/grade-4/
https://www.education.com/science-fair/fourth-grade/