Who didn't think that Far Cry Primal emerged remarkably soon after Far Cry 4? There were two years separating Far Cries 3 and 4 versus one between 4 and Primal, but Ubisoft assured us that Primal is a full-scale Far Cry game and not an expandalone like Blood Dragon. However, a discovery by Game Pressure suggests that Ubisoft had a head-start at the level design stage. The maps for Far Cry 4's Kyrat and Primal's Oros are extraordinarily similar.
It's possible that Ubi's map designers have pumped out so darn many of the things that they just forgot what wilderness they'd already developed: the main watercourse in Primal is more cohesive than in Far Cry 4, for one thing. However, the arrangement of lakes and tributaries and the overall layout of the biomes is so striking that it's hard not to see crafty recycling behind it.
Ubisoft could have blagged its way round the similarities if Primal were set in the prehistoric Himalayas, 12,000 years before the events of 4, but alas, it actually takes place in central Europe, as the many making-of videos have taken pains to highlight.
The fact that nobody noticed while playing is testament to the thoroughness of the conversion, so what happens in future will depend on how much people care. Would you feel duped if wholescale map reuse has indeed taken place, or do you simply not care given that it's only noticeable from an owl-eye view?