Yup, they taught me that rule of thumb too. It's not a specific law because the law says you must dive safely with appropriate following distance for road conditions, which can be longer than 4 seconds in bad weather.

Like my state has:

(1) The operator of a motor vehicle shall not follow another vehicle more closely than is reasonable and prudent, having due regard for the speed of the vehicles and the traffic upon and the condition of the highway.

It's both good and bad that there's no objective measure, depending on if you're the one getting a ticket for driving like an ass.