En avril 2006, le développement d'Half-Life 2: Episode Four est confirmé, marquant le début d'un nouvelle acte de l'histoire. De plus, cette extension serait programmée par un studio tiers placé sous la supervision de Valve Corporation.

HL2 Wiki :
'Half-Life 2: Episode Four' is an unofficial name for a 'fourth episode' said by Gabe Newell (in an interview in PC Gamer) to be in development by a third-party developer. The so called 'Episode Four' is apparently not strictly connected to the timeline or settings of the first three episodes, and may feature an entirely different set of characters / locales. Some have speculated that 'Episode Four' may see the return of Adrian Shephard or another Half-Life PC or NPC. However, as development on Episode Two and the Orange Box has progressed, details on the fourth episode have no been forthcoming, and it is not entirely clear whether the product is indeed in development any more.