The term “equity security” when used in this part means:
(1) Any stock or similar security;
(2) Any security convertible, with or without consideration, into such a security, or carrying any warrant or right to subscribe to or purchase such a security;
(3) Any such warrant or right; or
(4) Any other security which the commission deems to be of similar nature and considers necessary or appropriate, by such rules as it may prescribe in the public interest or for the protection of investors, to treat as an equity security.