Right, the 4g/day number is the amount that should be safe for any relatively healthy adult (minus whatever is making them take the medicine). I should hope it isn't too close to the number where you can cause permanent damage.
It's become fashionable to fearmonger about it lately, but I agree, it's safe to assume the makers wouldn't tolerate the liability that would come with a single extra dose above the given instructions causing that kind of harm.
I was surprised it was that low, but I think you misread/misremembered. From your link:
> Toxicity is likely to develop in adults at:
>12 g over a 24 hours, or 7.5 to 10 g in a single dose, or Doses 350mg/kg
The 4g/day is the dosage threshold, but it seems they set it to 1/3 of the toxicity threshold.