The release date means nothing.
Saints Row 2 has been released in 2008 (October).
Saints Row 3 has been released in 2011 (November).
25 months between the two episodes.
It Saints Row 4 is released in 2013 (August), it'll make 21 months between the two episodes.
So it's pratically the same amount of time. And between Saints Row and Saints Row 2 : 26 months.
It's normal that each version take a little less time, as you can always reuse the tools you used for the previous version.
The time of developpement also mean nothing about the quality of the game, GTA IV took a few years and for me, the game is rather bad (compared to the old ones).