As I previously mentioned, I really enjoyed Far Cry 3. It was far from perfect, having some very rough edges, but it stood as one of the most entertaining open-world shooters released at the time. The game was packed with content, from outpost take-over, tower-puzzles that unlocked bits of map (a staple seen in all recent Ubisoft games), to animal hunting for a brand-new wallet / weapon holster and a proper story campaign.
All of that returns, slightly expanded, as it is expected since this is an Ubisoft game. The tower puzzles have been turned into bell towers where Pagan Min is broadcasting propaganda, there are plenty of animals to hunt down, kill and craft holsters, bags, wallets and other exotic leather apparel, and finally, you can attack and conquer fortresses and outposts. The interesting bit is that the world feels a helluva lot more dynamic, with enemies attacking that which you have conquered, as well as Karma events, random situations that involve a friendly party and an opponent, whether that's a horrifying beast or Pagan Min's soldiers, remains to be seen.
Of course, there's a progression system in place, with upgrades that grant more life, reduce damage from fire, stealth-take down kills, or the ability to ride elephants and more. Again, just like Far Cry 3, just expanded, and at times more dynamic. You could spend 100 hours in the game and still have stuff to do, even though the main campaign shouldn't take more than 12-14 hours to finish.
Lastly, one improvement I am more than thankful for is auto-drive. It seems Ubisoft understood Far Cry 3's cars handled like cows on roller-skates, and added the feature to simply drive from waypoint to waypoint on the road, allowing you to focus on shooting whomever is chasing you.
Far Cry 4 has 2 multiplayer modes available. A classic five-on-five asymmetrical pvp-mode with 2 different factions (Rakshasa and Golden Path) that feels slightly tacked on, with simple point control or objective-type matches, though I have to admit a lot of work was put in to balance the two factions, even though they play differently. I cannot say as much about the co-op. Another player can drop-in and help you in certain open-world sections, whether that is cooperating on taking a fort together or razing a new outpost. This could be entertaining, especially if you have with whom to play it with. Unfortunately, the partner can only stay for a time, and cannot aid you through any story issues.
It's worth noting that Far Cry 4 is not a particularly difficult game, relying more on the power-trip kinda sensation instead of providing a challenge. Co-op takes away even what little challenge there is, so keep that in mind when you want to play in co-op.