The definition of material is the standard SEC definition of material nonpublic information and is not inherently linked to any of the hypotheticals you listed, except indirectly. It’s basically, would this significantly affect the stock price if investors were to hear about it? Executives at a public company should already be used to making that judgment call, and the only new thing as to materiality is that employees leading cybersecurity investigations at public companies will have to properly inform those executives in a timely manner.

This rule is not telling companies they have to rush to determine materiality - in particular, time from discovery of the incident is not what the four days in the article headline are referring to. The rule simply requires them to determine materiality “without unreasonable delay” after discovery of the incident. Only after such a materiality determination do the four business days kick in as a notification deadline.

So, if the delay is reasonable, then by definition it complies to take that long. In particular, a delay that is necessary to avoid disrupting a proper investigation would very likely be considered as reasonable. The company might have to be prepared to explain why both processes couldn’t overlap if the investigation stretches on a long time and no materiality determination has occurred, but okay, that’s fair enough to require.

The point of the “without unreasonable delay” restriction is because, without it, companies would just refuse to determine materiality ever, or they might do it slowly enough that the evidence necessary to declare materiality will usually have vanished, or similar bullshit. It’s to forbid circumventing the intent of the rule by sticking one’s head in the sand.