Tor Button shows an IPv4 address for IPv6 connections
In Tor 0.2.8.1-alpha, Tor can bootstrap and connect to guards using only IPv6. (For a few releases, Tor has been able to connect to bridges using IPv6 only, and guards using mixed IPv4 and IPv6 connections.)
But the circuit display only shows the IPv4 address of the guard, even if the connection is over IPv6.