These sites can be used to help your child study their multiplication and/or division facts:
ABC Ya has free games that are sorted by grade level and subject. This link should take you to grade 4 math games, and there are several that practice multiplication and division.
This website has games to practice math facts as well. You can set the "range" (I would start with 0-10 for most kids, but you can always start with 0-5 if you want to give more practice) and you can set the speed to slow, normal or fast. This link has the multiplication games at the top.
This website allows you to generate multiplication worksheets and has other printable resources. This link takes you directly to the multiplication page, but on the left side bar you can also navigate to a flashcard page and print flashcards for your 4th grader.