§4 of the 25th amendment makes reference to "a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide" – Congress has not provided anything else. 5 USC 101 lists the "executive departments". Art II § 2 of the Constitution says that the president
shall nominate, and by and with the Advice and Consent of the Senate, shall appoint Ambassadors, other public Ministers and Consuls, Judges of the Supreme Court, and all other Officers of the United States, whose Appointments are not herein otherwise provided for
It is reasonable (but not self-evident) that having been nominated to be a principal officer of, e.g., the Dept. of Justice does not thereby make you one.
What legal precedent can be called on to decide if an acting department head "counts" as a principal officer for purposes of the 25th Amendment §4, either for determining what number constitutes a "majority", and for determining whose written declarations are to be considered?