The first Left 4 Dead was developed by Turtle Rock Studios during their brief time as a Valve subsidiary. At some point, they were working on a fifth campaign that was intended to bridge the gap between Dead Air and Blood Harvest. Players start at the landed plane and work their way toward a hydroelectric dam. One section, contrary to the game's theme, was apparently designed to encourage survivors to split up, some in a sniper tower and others running through a trench.
https://www.pcper.com/
Link to the mod:
https://talk.turtlerockstudios.com/t/tr ... ign/106720
Gameplay:
https://www.youtube.com/watch?v=cgQ3dDwNRJs