Saints Row IV tries to expand on its predecessor by adding new content, new types of missions, new mechanics but keeping too much of everything else as it was.
It seems too generous to call IV a separate game, since it's pretty much more of the same. There are some additions, mostly to do with plot devices and gameplay. Notably, superhuman abilities make things feel more like Prototype than SR3 and add a different layer to moving through the city. Other than that, the action still revolves mainly around driving and shooting, both core mechanics that get ironically dropped to the background since no one wants to shoot a gun when they can throw a fireball or take a joyride when they can run like The Flash.
The fourth installment in the franchise doesn't really add anything incredibly innovative to the table, but if you enjoyed Saints Row: The Third, you can think of this as one huge DLC. The question you need to ask yourself is if you're willing to pay full value for a game that's pretty much identical to its predecessor plus some gimmicky features.
Sure. The main storyline is still whacky, fun to play through and mission design is as good as ever, but sidemissions (which still make about 75% of the content, mind you) get extremely repetitive after a while, as they did in SR3. On the other hand, there's now a cooperative mode and some co-op-only missions that might add some fun if you have a buddy to team up with.
There are also other, somewhat minor imperfections like texture popping due to the old engine not having been fully upgraded to handle the now large-scale movement, graphical glitches, subtitles not matching the script at all and the occasional crash. Mostly, they're not a huge deal but do give out a very unpolished and unfinished feel to what should be a platinum game.
As a final note, there are many references to the previous games in the series, so, if you're a hardcore fan, you might get off on the nostalgia of seeing some old characters make a reappearance.
The bottom line is Saints Row IV trying to cash in on what made Saints Row: The Third great, adding a Matrix-esque twist on gameplay but in the end not having enough substance to warrant paying 50 bucks for something that's pretty much a modded version of the previous game.
