At the time, Far Cry was said to have had a pretty good story, though that might have been a bit undeserving since, with the exception of a few stellar moments, the game was really not well written.
The characters were interesting, though questionable, with Vaas leading the charge. Sadly, he got killed halfway through (spoiler warning!), and replaced by a bland stereotypical trafficker. His sister was also its own brand of crazy, as was Doctor Earnhardt, but except these 3, everything was underdeveloped. The protagonist, Jason Brody, was a very generic type of rich douche who slowly sank into being a monster, but unlike Spec Ops: The Line's Walker, Jason seemed to enjoy it. Hey, we did too when we were burning all those weed-crops and shooting stabbing bad guys clad in red.
Far Cry 4 seems to fix many of the writing issues Far Cry 3 came with, but also introduced new ones. The story opens up with you playing the role of Ajay Ghale, returning to your mother's Homeland of Kyrat, to deliver her ashes atop Lakshmana. In the first fifteen minutes, through an unskippable sequence of cut-scenes (Ubisoft, seriously, why are custcenes unskippable?) you are introduced to the charismatic and brutal King Pagan Min, and the terrorist / freedom-fighter opponents, the Golden Path. Being caught in the middle, you discover that while you were raised in the US, you have a knack for destruction, hunting, hand-gliding, shooting while zip-lining and riding elephants.
Far Cry 4 also tries to one-up Far Cry 3 with its characters. If Far Cry 3 had a handful of crazy, yet fascinating or charismatic cast, then Far Cry 4 must have everyone be up the wazoo. As such, everyone you will encounter has, let's say, exaggerated character straits that make them stand out. The only one who doesn't is Ajay Ghale himself, who speaks even less than Jason did in Far Cry 3. He does have a few lines here and there, but they don't extend any further than why, what am I to do, and It shall be done. Humor is also a lot more predominant in FC4, which is only a plus, since it balances out the rather grim twists the game throws at you. While the grim tone is not as problematic as it was in Far Cry 3, it's still there taking you out of hunting strange folk atop snowy mountaintops.
Much of the story is available via a series of campaign missions provided by the various characters the game has, and you are free to blaze through them in any order. When you start a mission though, all that sandbox freedom is thrown at the window, and the game slaps you with a "You exited the mission space" warning before restarting the damn thing again.
During the campaign, you will have some decision moments coming from the two co-leaders of the Golden Path. The game will put you to back one over the other, changing the way said missions unfolds. However, this doesn't have a major impact in the game's outcome, it just makes subtle modifications to how you have to complete missions. Make no mistake, hidden under the freedom to take whatever you desire from the map rests a very linear storyline.