Download on Saints Row Mods
This mod is a little different from the others on this list. What it essentially does is gives you the ability to revisit older areas in the game that you normally wouldn't be able to return to. It comes with several designated teleport points that you can jump to with a few key presses. It even gives you the ability to hotkey in your own custom teleport points.
The creator of this mod has a great deal of renown in the game's community -- so much so that he was hired by Volition, the developers of Saints Row, themselves. That, plus the amount of freedom this mod gives you, makes it well worth checking out.
Due to the relatively small size of the community, finding good mods for Saints Row 4 can be difficult. But that shouldn't dissuade you from checking out sites like Steam Workshop, ModDB, NexusMods, and Saints Row Mods for more awesome fan creations like these if you're interested. You might just stumble on something great!