Nope, that was not me making a typo, I am actually referring to Episode 4. What do you mean you haven't even played Episode 3? All joking aside, Episode 4 was to be done in collaboration with Arkane (most famous these days for Dishonored) and set time-wise near the end of Episode 2 with the main character (perhaps even Gordon?) returning to Ravenholm.

For various reasons this project never came to be but traces of it still keep surfacing every few years. The folks over at Valvetime have once again unearthed new screenshots of Episode 4 which you can check out either through their website or the video embedded below.

And if all you're interested in is a quick overview of what the Episode would've felt like here's a few screenshots:

The rest of the screenshots follow a similar theme of abandonment and decay. Not to mention that the whole bloody thing seems to be set in a sewer! Because, obviously, when you're making a new game and you're brimming with ideas you start off with the crowd favorite, the sewer level. Still, imagine if the whole place was filled with headcrabs and zombies, that would be one very spooky sewer.