To INSTALL FS2004 to your new Windows 10 or Windows 11 PC
1. Install into C:\ NOT into it's default C:\Program Files
2. Install the FS9.1 update
3. Install the FS9.1 nocd patch (Replace the existing FS9.exe)
4. Right-click the FS9.exe, select Properties
5. Select Compatibility mode, & select XP
6. At bottom, then select 'Run as Administrator'
That should do the trick!!
Once you have done that, there is no need for the Disk 4 at all.