Section 4 addresses the dramatic case of a President who may be unable to fulfill his constitutional role but who cannot or will not step aside. It provides both a decision-maker and a procedure. The initial deciding group is the Vice President and a majority of either the Cabinet or some other body that Congress may designate (though Congress has never done so). If this group declares a President “unable to discharge the powers and duties of his office,” the Vice President immediately becomes Acting President. If and when the President pronounces himself able, the deciding group has four days to disagree. If it does not, the President retakes his powers. But if it does, the Vice President keeps control while Congress quickly meets and makes a decision. The voting rule in these contested cases favors the President; the Vice President continues acting as President only if two-thirds majorities of both chambers agree that the President is unable to serve.
Section 3 and (especially) Section 4 are long and complicated by constitutional standards. Nevertheless, they leave a number of issues unsettled—most significantly, what counts as presidential “inability.” At the Constitutional Convention in 1787, delegate John Dickinson asked, “What is the extent of the term ‘disability’” in the proposed presidential succession clause, “and who is to be the judge of it?” No response is recorded. By giving the President, Vice President, and Congress important and distinct roles, the Framers of the Twenty-Fifth Amendment went a long way toward answering the second part of Dickinson’s question, rather than try to resolve the first part.
https://constitutioncenter.org/interactive-constitution/amendments/amendment-xxv