Yes, I know, I know. All of you started laughing when you read that title.
But believe it or not, an Episode Four featuring a stand-alone plot was in development at some point. Not at Valve, but at an undisclosed third-party studio. PC Gamer mentioned this way back in May 2006, but in late 2006 Gabe said that this Episode Four was still in development. When talking to Eurogamer in May of 2007, asked about Episode Three’s setting, Doug Lombardi said: “I don’t think you’ll head back to City 17, for sure. Not in Episode Three, anyway.”
Strangely enough, as late as December of 2007, Stuff We Like, after speaking to Doug Lombardi, mentioned the possibility of a series of episodic Half-Life games disconnected from the main story arc. But, some time later, the project was cancelled. We don’t know when or why, and we don’t even know who might have been making it. Except now we might know!