Section 4 of the 25th amendment provides:
Whenever the Vice President [transmits a declaration] the Vice President shall immediately assume the powers and duties of the office as Acting President. Thereafter, when the President [transmits a declaration], he shall resume the powers and duties of his office unless the Vice President [transmits a declaration] within four days [...]
This seems a little ambiguous to me. On the one hand, the language suggests that the P resumes their powers as soon as their declaration is made. At that moment in time, it is uncertain whether or not the VP will transmit a 2nd declaration, so the "unless" qualifier doesn't yet apply. On the other hand, if the VP does eventually re-declare, then the "unless" is contradictory if the resumption of power has already taken place.
- Day 1: VP makes 1st declaration.
- Day 2: P makes declaration.
- Day 3: VP makes 2nd declaration.
On day 2, there has been no 2nd declaration, so the "unless" provision isn't (yet) applicable and there is nothing to stop the P resuming powers. But on day 3, the "unless" provision is invoked, meaning that no resumption can take place. But it already has if the earlier interpretation is correct.
My guess is that any resumption is effectively on hold until either the full 4 days elapses, or the VP re-declares. Any other views or authority?