Most bear markets include periods when stocks drop sharply and quickly over a period of a few weeks to a few months
Mid-1970, late 1974, late 1998, and several episodes during the 2000 -2002 bear come to mind. I think Hussman is referring to the market being vulnerable to these types of drops only 4% of the time. A 1987 type crash is as you say only a once or twice a century event.
“The things that will destroy us are: politics without principle; pleasure without conscience; wealth without work; knowledge without character; business without morality; science without humanity; and worship without sacrifice.” Mahatma Gandhi