If you haven’t played the previous Saints Row games, keep in mind that Saints Row IV uses a lot of references from the previous games.
I had only played Saints Row The Third before, and if I knew about this, I would’ve first wanted to at least play Saints Row 2 before playing Saints Row IV. Not only does Saints Row IV contain characters and information I’m not familiar with because they are from the previous games, some story information from Saints Row 2 now has been spoiled for me.
Should you not care about any of this, by all means do go ahead and play Saints Row IV. You do not need to have played the previous Saints Row games in order to enjoy this game, but I personally think it’s nice to know the game series, the stories and characters.