I'm playing through Far Cry 3 at the moment. Open world is beautiful and you definitely won't feel locked down into a particular path. Many points of interest which makes exploring around fun and rewarding. Fast travel system in place but running and driving around is a pleasure. Lots of collectables which you may see as a + or a -. Stealth and detection system is sort of meh but it's a viable option for taking over outposts and rewards you with exp bonuses. The skill system is interesting and sort of gives you paths for specializing.
Tips:
I wouldn't bother with plants and the medicine things they are used for.
Unlock all 4 weapon slots as soon as possible.
Get a silenced sniper with a decent scope.
Animals hate you.
Crocodiles are terrifying.

You'll have fun with it.

As for 4 I haven't seen much other than the trailer and the gameplay sneak peak. The location looks very nice and I'm intrigued by the supposed bad guy and what the story will be. Hopefully Ubisoft doesn't nerf the PC version like they did with Watchdogs. xD