You can vote until 11pm, can't you, so if they're not staying off air until then what's the point?
And this is not an original idea. Nickelodeon are the originators. They have a campaign in America, where the channel goes off air for three hours to encourage kids to go out and get some exercise. It's called Worldwide Day of Play (despite not being worldwide nor all day).
It's such a horrible, self serving & cynical stunt. If E4 cares so deeply about youth and democracy where's the programming attempting to engage their audience in politics?. Nowhere to be found on E4. Doomed BBC3 however has some excellent youth skewering political/current affairs shows that have, more often than not, gone overlooked by critics of the channel who preferred to scapegoat Snog, Marry & Avoid.
If you want to grab the attention of E4's demographic, and get them to go outside and do anything, you need to switch off their internet access. It works in our house.
As already mentioned it's more a publicity stunt than anything, as they're only going off air until 7pm. They're not staying 'off air' until polls close at 10pm which would mean being off air during prime-time. Still fair play to Channel 4 for doing something different.
E4 right now, has the E4 guy playing Noughts and Crosses on a small blackboard, with someone dressed as a clown. A spoof of the BBC Test Card! Brilliant...