You can purchase it directly from within the police precinct, or via the episode page here:

https://steamproxy.net/steamstore/app/740591/The_Exorcist_Legion_VR__Chapter_4_Samaritan/

• Chapter 1 is required, and this story continues where Chapter 3 left off.
  
  
• This chapter requires you to think. Take your time, and explore. There are details, hints and backstory elements hidden throughout the chapter. Some of this information might come in handy when it's time to make decisions.
  
  
• Use Father Bell's notebook in your inventory! The information inside is valuable.
  
  
• Use your phone to adjust your locomotion settings for free roam, free turning, or all manners of teleport. Oculus Rift users can change their settings to accommodate 360-degree tracking.
  
  
• When you return to the precinct at the end of the chapter, check out the evidence room.

Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me...