Fourth grade science projects should be fun and interesting to hold the child's attention. Fourth graders can carry out the experiment pretty much on their own, but they will likely need help coming up with an idea and then beginning the project, and also with presenting their findings at the end. In fourth grade, science projects generally involve answering a question, solving a problem or testing a hypothesis.
There are countless ideas for fourth grade science projects, and one interesting one is to see if seeds will grow if they are watered with a liquid other than water. All you need for this experiment is some seeds, a few small pots, some potting soil and a few different liquids like milk, juice and vinegar. Simply plant a few seeds in each pot and water it with whichever liquid you have not chosen, and then repeat this with the other pots. Be sure to label each pot so you know which liquid to water it with. Then, watch for the seeds to sprout over the next few days and record your findings.
You could also do a similar experiment and see if plants will grow if they are watered with soapy water, such as leftover dish water or bath water.
Another 4th grade science project that might be fun to try is to see if cockroaches have a preference for direction. To do this, you simply need to catch some cockroaches and find a spot outdoors to set them down and record which direction they head in. Is there a common trend or not?