Bob has provided part of the answer. I would like to add that you will not really help your child if you create his or her hypothesis, etc., for him or her. Your role as the parent would be to ask about what he or she remembers being taught about doing the project. Provide some prompting questions and help him or her find a question or a problem to address that is interesting, and then help him or her to find some resources (and Bob is right, those education.com links provide a lot of good information on science fair projects).
Kids (and even teachers) often confuse hypotheses with predictions. Many projects at the 4th grade level are really tests of products, like the ice cream vs yogurt melting idea of another parent on this site). Hypotheses are often stated in "if..., then..." language while predictions are much easier to understand and state.
As a professional science educator, it is my opinion that teaching experimenting in the 4th grade is developmentally inappropriate as many children at 9 or 10 have not developed sufficiently for the kind of mental prowess required. Schools have pushed the curriculum down lower and lower in an attempt to beat the testing regime.
This website (Education.com) has a lot of articles and project ideas for science fairs. See the links below. The first takes you to a collection of articles ABOUT science fair projects and the second to a list of project ideas for elementary school students.
Your "hypothesis" is the theory that you are trying to prove or disprove by the experiment (for example "increasing salt in a bread recipe has no effect on how much it rises"). Your problem statement is a short description of the method you are going to use to prove or disprove the hypothesis - that is, what you intend to DO and how it proves/disproves the theory. The title is often in the form of "The Effect of (fill in the blank) on (fill in the blank)" as in "The effect of salt on bread rising" or "How salt affects bread rising."