Four minute nuclear attack warning

In the 60s, we were told what we should do in the event of a nuclear attack (it was openly assumed that any such attack would be coming from Russia or other countries in the USSR). I seem to remember that we were told we would have 4 minutes' warning of an attack (which, of course, resulted in endless jokes about how people would spend those few minutes) during which we had to whitewash the windows and get under a table or into the under stair cupboard. When we left the house, we had to wear stout shoes and take blankets. That was for the general population; for important government folk, deep and well stocked shelters were in place.