Cannot recover previously bought app
It looks like the developers have decided to isolate each new version to be an app in its own right.
Owning v1, 2 or 3 does not automatically get you v4.

https://itunes.apple.com/gb/app/goodreader/id777310222?mt=8


  The amazing 4th edition of GoodReader® app is a universal app for all your iOS devices. 
  
  This version is a further step in development of our original groundbreaking, best-in-class GoodReader app. If you're a user of the old GoodReader app, we welcome you to upgrade to this 4th edition of GoodReader. 


This would mean that unless you have a local backup of the app, it would appear it is no longer on the iTunes Store to re-download.