Far Cry Primal launched only one year after the Far Cry 4 and I think we have discovered one of the reasons behind it.
Thanks to Game Pressure, we can see that Far Cry 4‘ Kyrat and Far Cry Primal‘s Oros maps are oddly similar. They compared two maps by marking water reservoirs and rivers. Simply it looks like Kyrat just got a little smaller and transformed into Oros. You can check the maps below:
FAR CRY 4 MAP
FAR CRY PRIMAL MAP