I'm not really up on Copyright laws, however, I believe that CDs that you purchase can be copied if you are using them for your own personal use. So I believe this is OK. So Ozzy and Fly2e please don't ban me!!
The file that Politically Incorrect is probably referring to is called the "No CD Patch for FS 2004" and is available here:
http://www.simviation.com/fs2004misc1.htmThis replaces the FS9.exe file in your main FS 2004 folder with an updated FS9.exe which does not require the install CD disk #4 to run the FS 2004 program.
As Politically Incorrect was explaining this "patch" only works with FS 2004 installed from certified Microsoft FS9 CD disks. I would also like to second Politically Incorrect's motion and caution you to "use it at your own risk".
What you may consider doing before going this route is to rename and restore the FS9.cfg file. See if this cures your issue.
To do this with Windows XP use the Windows Explorer to Browse and Open the C:\Documents and Settings\Your Name\Application Data\Microsoft\FS9 folders.
Look in the FS9 folder for a file called FS9.cfg. Rename it to FS9(Old).Bak [or whatever -- just make sure that the file extension is changed to *.Bak].
Restart FS 2004.
What this does is to force FS 2004 to start out as if you newly installed it. Now you will have to reset all of your preferences and sliders, however, this is a lot less time consuming than reinstalling or taking the chance that the "No CD Patch for FS 2004" is going to mess up your hard disk.