Macmillan/Mcgraw-Hill and Harcourt School Publishers are two reputable places to find fourth-grade textbooks online. Another place is a website called Eduplace.com. These three sites are only useful if you have a username and password to access the textbooks.
Schools often provide students with access to online textbooks through a publisher or website. Some schools, such as New York City Public Schools, post their username and password online for public use. Other schools might require you to procure a student id number. If a student is unable to access from her school, a parent can register the student as a private individual on Harcourt.