I do not know how they came up with the 2-4 second following distance recommendation. As you point out, this is in line with what the UK recommends as well. Probably it is a compromise between safety and practicality.

> It would have to be further than braking distance to be at least as safe?

Other way around. Following distance can be less, because the guy you are following cannot stop instantly unless he hits an immovable object or gets into a head-on crash. If he panic stops, then as long as your car performs similarly in braking you just need to have enough distance to allow for your own reaction time.

AFAIK braking distance for most cars is around 5 seconds at highway speed. Few people routinely set their following distance that long.