Far Cry Primal appears to recycle several parts of Far Cry 4‘s world map — the two shows striking similarities in terms of river and landmark placement.
Gamepressure has published both maps side-by-side and highlighted the similarities between the two, which seems to show a vast portion of the map has been lifted. The site noted that some slight alterations have been made, such as connecting reservoirs of water to streams, but apart from that, the framework is pretty much identical. You can see the images below.
FAR CRY 4 MAP
FAR CRY PRIMAL MAP