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.
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Turtle Rock Studios has apparently streamed gameplay at some point. Since then, they published the campaign as a Left 4 Dead mod. It's not finished, and apparently quite glitchy, although I haven't played it yet, but it's free.
Check out the post at their forums, which has a link to concept material and the mod itself.