Blood Harvest

Blood Harvest is the fourth campaign in Left 4 Dead. The Survivors must travel to a barricaded farmhouse and get rescued by the United States military. The escape vehicle is a United States military armored personnel carrier.

Here's a few gameplay videos showing walkthroughs for chapters 1 to 5:

http://www.youtube.com/watch?v=P6sMEbk6VJI

http://www.youtube.com/watch?v=-zZZGBehSdg

http://www.youtube.com/watch?v=rc72G1QPRKc

http://www.youtube.com/watch?v=wnDporSUiuI

http://www.youtube.com/watch?v=TFktt86pTbc

Setting
As revealed by a park sign at the starting location, this campaign takes place in Allegheny and around Allegheny National Forest in Western Pennsylvania, a couple hours' drive from the city of Pittsburgh. Pittsburgh (and its surrounding region) is famous for providing the settings to many films such as Night of the Living Dead and Dawn of the Dead.

Official description
Zombies lurk behind every tree, over every hilltop and behind every shadow in this terrifying moonlit search for a rumoured evacuation zone to the north: several acres of back-country farmland taken over by the military to get the last of humanity out of Ground Zero. Plunge through pitch-black forests, stumble blindly through fields of swaying corn, and take aim at the silhouettes leaping at you from the darkness—all towards finding a hold-out that might already have been abandoned to the horde. Zoey can be seen in the same position as she is in on the No Mercy campaign poster, except not crouching.

Writings on the wall
In the safe rooms, the writings on the wall are telling the remaining Survivors to follow the train tracks to get evacuated to the safe zone. Additionally, when walking to the farm where the APC will arrive, there will be two signs on a post. Both appear to be from the military. One says "Keep your hands above your head in the field at all times or you will be shot on sight!" while another says:

1.Do not run at military personnel.

2.Do not ignore personnel when they address you.

3.Do not make any violent or sudden movements.

These signs are to help the military tell who is infected or not.