Article II section 4 would be superfluous if the head of the executive branch and the nation’s chief magistrate could be subject to prosecution under Article III. The beautify of the Constitution is its simplicity.
“Article II section 4 would be superfluous if the head of the executive branch and the nation’s chief magistrate could be subject to prosecution under Article III.”
That doesn’t follow at all. A president could be prosecuted for acts done before becoming president, which would not be a basis for removal from office. Or a president while in office might commit an act that was a crime but that didn’t rise to the level of “Treason, Bribery, or other high Crimes and Misdemeanors.” (That phrase must mean something other than just any old crime.) Or a president might do anything at all on January 19, when there’s not enough time to go through impeachment and conviction.
The Constitution specifies the bases for removing someone from office. It doesn’t say that removal from office is a necessary prerequisite to an ordinary criminal prosecution, especially one brought after the president has left office anyway.