Escenkay, you also need to remove the LaunchConditions from your cache installation file.
You will want to search your registry (regedit) to determine which installer package to update for BC 4.
Run "regedit" and do a search for "
www.apple.com/support/bootcamp"
You will find it in Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\...
You should find an entry for "Boot Camp Services"
What you want to look for is "LocalPackage" which should point to "C:\WINDOWS\Installer\xxxxx.msi" where "xxxxx" is some random file name. Modify the launch conditions in that msi.
The reason why you are seeing the "Windows 7" message when trying to install BC6 is because the installer is trying to uninstall BC4.
Another location for the BC4 installer cache is in "C:\ProgramData\Apple\Installer Cache", but I didn't find making changes to that making any difference. The one you want to modify is in "C:\Windows\Installer"