On the game's official web site you can read that it will be possible to take part in Far Cry 4's co-op missions even if you haven't bought the game. Of course, you need to be invited through PSN by a friend that actually did buy it.
This Playstation exclusive feature was explained in a little more detail when Gamespot spoke with executive producer Dan Hay.
PS3 and PS4 owners with PS Plus will be given ten invitations to play "a free trial of online co-op with you. Explore Kyrat together, capture outposts, gain money and experience, and create your own stories with a friend."
"We're going to be generous. We want you to basically download the game, jump in your buddy's game, play as Hurk, play for a couple hours, run around, experience the entirety of the world, take over some outposts, take over some fortress, understand what it's like to be able to wingsuit, to be able to climb on top of the elephants, to be able to get a whole bunch of new vehicles and weapons, and effectively play the game," Hay said.