Section 4 of the 25th Amendment seems unclear to me. It states that once the VP and executive members submit in writing to President Pro Tem and Speaker of the House that POTUS is unable to fulfill his duties then the VP assume the role of acting president.
POTUS may then in writing state that he is able to perform his duties and resumes his role as POTUS unless VP and executive members state again that he is not within four days.
Senate and House then have 21 days after convening to decide if POTUS is able to discharge his duties. General thinking suggests that the language of 25th Amendment implies that VP "continues" as acting president; whereas, POTUS "resumes" his duties if Houses decide he is capable.
It seems clear that the VP continues as acting president once second letter is submitted but who is in the office during the interim between POTUS letter and second VP letter?
Does POTUS resume his duties immediately upon submission of his letter and remain in that capacity until the VP and executive members submit their second letter? or does POTUS wait four days to see if letter will be submitted during which VP is acting president?