Ps4 – Is it possible to play Far Cry 4 co-op with players on other systems

The game's cover states local players to be 1-7 and network players to be 2-22 (zoom in on the image to look closely).

Since the max controllers supported at the same time on a single PS3 is 7, online support is at least for 4 consoles (3 consoles * 7 players + 1 console with 1 player or similar configurations). I don't know the exact number of consoles supported.

Try checking the game's manual (Warning: PDF) for further info.

Sony allows to have people in other platforms to interact with their players but MS don't.

One of the recent cases in last year that explained that to the user was Square Enix, regarding questions of why not having Final Fantasy XIV on Xbox.

Their answer was that since Microsoft has an strict policy concerning online servers, the xbox users would had to have their own private server, with no possibility of people transferring from/to and therefore SE gave up on the idea of porting to Xbox.

Check the IGN interview with SE's Naoki Yoshida for more info on this example:

"The policy has not changed on Microsoft's side," he revealed. "The main reason from our side is that I don't want the community to be divided; to be split into two or more. For example, one player might be on the PC version, another might be on the PS4 version, and I'm playing the Xbox version - but we're not able to join the same game servers. That is just... I just don't like the idea. I disagree with it."