If not Article II, Section 4, then using Section 4 of the 25th Amendment

As the president’s most tireless online antagonists repeat hourly, Trump would seem to be a prime candidate for impeachment and/or removal from office, if not under Article II, Section 4, then using Section 4 of the 25th Amendment, which provides a means of removing a president on grounds of impairment (“unable to discharge the powers and duties of his office”).—http://theweek.com/articles/698288/theres-no-waking-from-americas-trump-nightmare