David Cairns, Scotland Office minister, has resigned, saying
The issue of leadership and direction are being discussed and argued over, and to go on denying it is hardly credible.
To that end I believe that the time has come to take the bull by the horns and allow a leadership debate to run its course.
I know that it is incompatible to hold this view and to remain a serving minister, and although it had not been my intention to resign, I have reluctantly concluded that it is the only honourable course of action left open. (BBC)
For details of the three previous departures in recent days, see here.