This is Night Owl. Do not question him.

You may notice I called the footage files "demos" before. These are not game demos in the common sense, nor are they even footage in the common sense, but rather, "maps" that tell the game exactly what happened during a particular time. Everytime a demo is played, the game literally recreates what happened that fateful day. This means that demos take up relatively little space on a hard drive, but also that they're somewhat limited as footage. You can't upload them to YouTube, edit them in an applicable program, etc. In fact, you can't even watch them ingame if L4D has been patched beyond whatever version it was when you originally recorded them! This sucks, I know.
However, just as you can record the demos, you can also convert them to a more compatible and long-lasting format. Namely, AVI. Just follow these steps...
1. If you don't already have it, get the appropriate version of the XVID codec from the following site:
http://www.xvid.org/Downloads.15.0.html
2. Open L4D to the main menu.
3. Set the resolution and general quality to what you would like the finished movie to be at. Several notes here...
- First, you may set your video settings to pretty much whatever your card's capable of. FPS is not an issue when converting your demos, and higher video settings generally won't affect the finished product.
- Second, set your resolution to something appropriate. Higher resolutions will look nicer, but take up a lot more space on your hard drive, while lower ones will look less nice, but cost less memory. Also, keep in mind YouTube currently has a 15 minute, 2GB limit on videos, meaning you can't exceed certain specifications if you want to upload them at some point. I generally find that a resolution of 800x600 works well.
- Third, if you have captions turned on on a lower resolution, you may want to turn them off for now. Captions have a way of dominating the middle of the screen in lower resolutions (like 800x600).
- Fourth, whatever you do to your settings now, keep in mind you can always change them back once your video is finished converting.
4. Now that you've done all this, it's time to start the actual process. Bring up the console and enter "startmovie [movie name] avi", without quotes, of course. Hopefully you knew that before this point...
5. A dialog box will pop up. Select "XVID Mpeg 4" from the drop-down list and press OK.
6. With the console still opened, type "playdemo [demo name]" in, and press enter.
7. Wait for the demo to finish its playback. It's essentially a slower version of the entire game you played, so this may take a while.
8. Once the demo finishes, type "endmovie" into the console (it should be opened still), and wait for the load to finish...loading.

And there you have it! The movies will be stored under the filename you chose in your L4D base folder (typically "C:\Program Files\Steam\steamapps\common\left 4 dead", remember). You may now do whatever you like with them.