There is a way around it. First, make sure you save your Maya 5 file as a .MA file. Then, open the .MA file in a text editor. At the top of the file, you can change the Requires attribute from 5.0 to 4.0. Save the file. You can now open it in Maya 4. Keep in mind that this may not always work, depending on what you're doing in the scene file. Some things are specific to Maya 5 and are not backwards compatible.