When you apply an SBS Client Add Pack to SBS versions 4.0, 4.0a, 4.5, etc.., you receive:
Your computer is not running Microsoft Small Business Server. You can only add licenses to a computer running Small Business Server.
You probably have a missing or corrupt Version registry value at:
This key should have a Version value entry, a type REG_SZ value, that must reflect the installed SBS version.
If you have SBS 4.0 installed, set the Version string value to 4.0.