3.8.3.17 -4 and -6: specify an Internet protocol version

The -4 and -6 options force PuTTY to use the older Internet protocol IPv4 or the newer IPv6 for most outgoing connections.

These options are equivalent to selecting your preferred Internet protocol version as ‘IPv4’ or ‘IPv6’ in the Connection panel of the PuTTY configuration box (see section 4.13.4).