“and I’ve never read or heard this 4 minute number before”
Because it’s top secret, or at least it was.
TECHNICALLY it might not even be (or need to be) that long. Slide the top off a silo and whoosh, out it goes — the solid fuel means it is always ready, and its control systems are up and running 24/7. This might just be some programmed delay to recall the launch for tactical sake.
Not sure also that there is not some secret code or signal that would cause the launch to down itself. Of course that is all in the realm of speculation.