The action in FC4 totally chaotic especially with the animals which are numerous and hyper aggressive. There are far more side missions and you can replay any single Outpost or Fortress as many times as you like. You have the Buzzer (gyrocopter) and the Himalayas missions are fantastic, as are the Shangri La missions. Overall it is a very intense game. That said , I would say FC3 and 4 are equal but different.
^This, also you can buy the wingsuit as soon as you have enough cash (and there are way better places to use it now). There's elephant riding (they can swim with you on their back too), and there's a grapple you can use to assist in cliff climbing and rappelling. In fact one thing that's cool about the grapple is there's some great momentum sequence swings here and there, and puzzles to get to special spots or just unique ways of getting to what are usually easy entrances, like towers. There's also Autodrive now, which on main roads acts as a chauffeur and lets you shoot hands free, or just enjoy the scenery. There are also traveling Sherpas, from whom you can buy anything that's available at trading posts. There's also a morale system called Karma and random events everywhere that unlock Karma points by assisting fellow Golden Path members. One of the best differences though IMO is boss fights are way better now and don't use cheesy QTE knife fights like in Far Cry 3.