There is no way to disable IPv6 in Opera. To use Opera without IPv6, it is necessary to turn off IPv6 at the operating system level (which is typical browser behavior).
Opera preferred IPv6 unconditionally over IPv4 until March, 2010, when Opera started using the operating system's instead of its own built-in resolver library.
To be certain that the IPv6 protocol is being used to access a website, substitute an IPv6 literal address surrounded by square brackets in place of a domain name:
http://ipv6.test-ipv6.com
becomes
http://[2001:470:1:18::115].
For additional information, refer to Request For Comments (RFC) 3986 “Uniform Resource Identifier (URI): Generic Syntax".
(Caution: This substitution can sometimes fail. Explanations of possible reasons for this are available. If you encounter problems, review the Broken User FAQ article found on that website for several possible explanations. For even more possible explanations, review this article on the ARIN IPv6 wiki.)
Extensions: IPvFoo is one of several extensions now available for Opera on the Chrome Web Store. After you install the iPvFoo extension, some combination of 4, 6, and/or "?" will appear at the top of the screen near the menu button (
). Clicking on it will cause the IP address(es) of the web page you are viewing to appear. Adding an extension to Opera from the Chrome Web Store is documented in this article.