Most tor clients send a DNS name, and flags that say whether they
allow IPv4 and IPv6, and which one they prefer. They rely on the Exit
to resolve the IP address and connect to the site.
On the current network, an IPv6-only Exit won"t get the Exit flag, and
therefore won"t get much client traffic. And it probably shouldn"t,
until almost all internet sites are on IPv6. Otherwise clients will
ask it to connect to IPv4-only sites, and it will fail them.
But thats exactly the case in a "tor exit that can only be reached by ipv6, but reach itself ipv4 and ipv6".
Can such a exit be run at the moment? IPv6 can be used on such a exit for in+out-traffic, IPv4 can be used to reach out everything - its just behind a NAT.
IPv4-only sites can be reached from the exit. The only case is that the exit itself can only be reached over IPv6 because of IPv4-NAT.