Hey dny,
Does this 4.5gB use persist? Or do you get it back when the dvd burn is done? If the later, there is most likely nothing you want to do about it. Any burning software is going to create a disc image, or at least BMP and MPG files, somewhere on a hard drive, so that it can supply the info expeditiously to the burner, i.e., without waiting on your processor.
If the disc use persists, find out where the software is storing temp files, and look there for the undeleted culprits. You can change the directory where this happens by clicking on the "Options..." link on the bottom right of the main DVD Maker screen. Look for the field that says "Temporary File Location" in the window that opens.
If the problem is that you want these temp files created on your external hard drive, instead of your internal hard drive, you should be able to take care of that there too.
Note, that if you elect to save the project, these temp files will likely be moved somewhere inside the Users\"username"\Videos folder. Look around there to see if there are files you'd like to get rid of.
Cheers