The Aussie-developed Satellite Reign was a solid performer on release, though it's slowly gotten better with age. That's probably due to the fact 5 Lives Studios has been pretty dedicated with the updates and its latest patch, which includes four-player co-op, is no exception.
An announcement of the game's Steam page clarifies that the co-op feature is technically an "open beta", however the developer has integrated it into the main branch and expects to launch it properly by "the end of the week".
Co-op is supported both online and via LAN -- which should make fans of Syndicate Wars happy -- and other than a few niggling problems, looks like it'll be an excellent (and massive) addition to the game:
We were having issues for some time with random disconnections, as well as an issue where clients would suddenly see everything stop (e.g. agents not responding, enemies and civs standing still), but everything is fine for the host. We're fairly confident we've solved this issue, but if anyone experiences this, please let us know! It haunted our dreams for so long, we're almost finding it difficult to believe that it's fixed! We're also aware that you may not be able to see games hosted in other regions. We're using the Unity networking system, which uses their own relay servers, located around the world. Currently, games hosted in other regions don't seem to be showing up in the server browser—however games hosted by overseas Steam friends should be visible if you change the filtering to "Friends." We've contacted Unity about possible solutions.
The icing on the cake? Satellite Reign is 50 per cent off until the end of the month. Talk about a persuadertron to the face.
Co-op Now in Open Beta [Steam]