Sony has published a patent that talks of amending in-game difficulty levels in real time as a player perhaps struggles or succeeds with certain elements
Huh, I didn't know about that mechanic in that series. I never really did end up liking L4D much, and thinking back on it, probably my biggest issue with the game was a feeling that I never really improved. No matter what I did, every time I played felt like I couldn't catch up, and it wasn't an enjoyable, "challenging" feeling. It was just frustrating.