You will see an error:
"You are attempting to save a file type that is blocked by your registry policy settings." when trying to save as.
Save this as Excel2003WK4.reg and then run it
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Excel\Security]
[HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Excel\Security\FileOpenBlock]
"BinaryFiles"=dword:00000000
"LotusandQuattroFiles"=dword:00000000
[HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Excel\Security\FileSaveBlock]
"BinaryFiles"=dword:00000000
"LotusandQuattroFiles"=dword:00000000
I think the same for 2007 would be:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security]
[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security\FileOpenBlock]
"BinaryFiles"=dword:00000000
"LotusandQuattroFiles"=dword:00000000
[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security\FileSaveBlock]
"BinaryFiles"=dword:00000000
"LotusandQuattroFiles"=dword:00000000
but I'm not sure.
NB I was a bit annoyed to find Symphony and Open Office do not support WK4 files. : (
Thanks to Jez Greenwood for this.