What is the test for a cause of action under section 425.16 of the California Public Prosecutions Act?

Section 425.16, subdivision (b)(1) states that the statute applies when the cause of action arises from "any act . . . in furtherance of the person's right of petition or free speech under the United States or California Constitution in connection with a public issue . . . ." (Italics added.) Section 425.16, subdivision (e) defines this italicized phrase as including four categories. The first two categories pertain to statements or writings made before, or in connection with, a "legislative, executive or judicial body, or any other official proceeding . . . ." ( 425.16, subd. (e)(1), (2).) The third category involves statements or writings made "in a place open to the public or in a public forum." ( 425.16, subd. (e)(3).) The fourth category includes "any other conduct in furtherance of" free speech or petition rights. ( 425.16, subd. (e)(4).) The latter two categories require a specific showing the action concerns a matter of public interest; the first two categories do not require this showing. (Briggs v. Eden Council For Hope & Opportunity (1999) 19 Cal.4th 1106, 1117-1118.)