Interesting. I've never seen a 4hr rule until you pointed that out, hightree. The usual rule that I hear is 90 mins. I'll have to keep digging.
I said 4 hours, someone will say less. I personally think that sport drinks might not be needed even for a marathon. Of course someone wants to drink it because of taste and to get some sugar for energy. A woman who died after the Boston marathon in 2002 due to water intoxication, was drinking Gatorade. And the exact idea of sport drinks is to provide sodium and thus prevent water intoxication. But if you drink too much of that, you still get more water than sodium and you are in danger.