Rape under section 4 . 4.—(1) In this Act “rape under section 4 ” means a sexual assault that includes—

(a) penetration (however slight) of the anus or mouth by the penis, or

(b) penetration (however slight) of the vagina by any object held or manipulated by another person.

(2) A person guilty of rape under section 4 shall be liable on conviction on indictment to imprisonment for life.