Who said it was the main point?
You're gonna have an EMP bomb that affects about half of the globe. The internet will basically die, because even though some computers will still work, a lot of the servers they connect to are gonna go KERFIZTbzzzz.
Other points to consider:
There'll be a giant dust cloud; this will affect all solar powered objects, and it'll clog up wind farms. Hydro might even go out, if the giant cloud of crap decides to settle in unfortunately placed rivers.
There will be no food for foraging for many, many miles around the area of impact.
At the right trajectory, this object could strike earth in such a fashion as to kick something back into night sky, giving us a second, but smaller, moon. (But the force required for this might actually end all life on the planet instantaneously. Not sure.)
Maybe your object should be smaller? I've read before that human civilization would end if a *one* mile wide object slammed into us. Four might be overkill.
The resulting cloud cover the planet would suffer might throw off the biological clocks of noctural creatures -- or even humans. It'll be much darker, most of the time, and animals we aren't familiar with might start appearing, simply because they're confused too.
After the fall, there would probably be a lot of sand storms or dust storms in area with high winds.
I think having people refer to the time after the impact as "After the Fall" is really cool; if you like that, feel free to use it.
Some species might evolve relatively quickly to adapt to their new environment -- perhaps even people? Eyes would get much brighter, as brightly colored irises help people see in cloudy conditions. Noses might get flatter and would have more hair, to help filter dust.
Will the earth heat up, or cool down because of such an impact? I suspect it would cool down, as most dust won't act as a green house gas - but massive amounts of dead and dying creatures would release equally massive amounts of methane, which IS a greenhouse gas. Maybe the temperature would spike, and then slowly settle to cooler conditions?
Compasses probably won't work correctly for a while; could the impact site actually become a third pole?
If humanity survives the cataclysm, the area of impact would be a point of great interest -- the object may have dug far enough to reveal geo-thermal vents, ideal for collecting as an energy source. Alternatively, it may become a vast (freshwater?) lake.
That's all I got. Hope some of it is useful.