Volition and Deep Silver announced today that Steam Workshop support has been added to Saints Row IV today.
The companies said that there were many challenges to overcome when adding Workshop support, as the game was not originally designed to support modding. Only the Windows version of the game supports mods at this point, although they hope to add Linux mod support in the future.
You can access the workshop here. The game is also currently on sale on Steam this weekend. Saints Row IV is available on PC, PS3, PS4, Xbox 360, and Xbox One.