: Skype version 4.0 is over a decade old. Microsoft (the owner of Skype) has long since "retired" these versions, meaning they likely won't connect to modern servers.
: Older versions are often hosted on unofficial, third-party sites. These ZIP files can be modified to include spyware or viruses . Download skype 405 zip
: Running the app from a USB drive without installation. : Skype version 4