Developer Volition has added Steam Workshop and mod support to Saints Row IV, which was launched over three years ago.
Saints Row IV was never designed to support mod, but the developers over at Volition decided to dedicate their free time to updating the game’s engine and remove any memory restrictions that in the way of full mod support. Now Saints Row IV can support individual mods and allow multiple mods to interact and work correctly without any serious issues, though general mod incompatibility rules still apply.
As of now Steam Workshop support is only available to the Windows Version of Saints Row IV, though support is coming to the Linux version soon.
At this time there are not that many mods on the Steam Workshop for Saints Row IV, although this is still early days, right now there are already several clothing mods, power enhancement mods and unique weapon mods to use. You can check some of the mods from the Steam Workshop here.