Deep Silver have mentioned that co-op in the Australian release of Saints Row IV will not work with copies released for the rest of the world.
In a statement posted on the Saints Row Facebook page, it explains:
Due to the changes we were forced to make, this version is different than the version rated by rating boards like the ESRB, USK, and PEGI, which is why it will be incompatible with those versions in co-op.
Saints Row IV was initially Refused Classification in Australia due to an optional side mission which featured narcotics that presented a positive effect towards gameplay. The appeal was unsuccessful and a modified version of Saints Row IV was granted MA15+ late last week, with the optional side mission removed.
For this version, we had to cut one optional Loyalty Mission from the game. Loyalty Missions are optional side missions that you can do with your homies. This mission in particular involves alien narcotics in the simulation, which eventually have a positive effect in terms of gameplay. As you cannnot depict any positive effect for using narcotics in video games in Australia, this was the reason the game was refused a classification.
Saints Row IV is set to release in Australia on 22 August 2013.