I spent about 6 months straight of my teen years on the original Saints Row. To me, that's still the best one. The others have been good, but never felt as good as the original did. I much preferred when Saints Row was just a less-serious GTA, with a few far-fetched happenings. Not the completely over-the-top, all the time thing that started in SR3. Saints Row 1 had much more drama than 2 and 3 put together, but still had the same sense of humour along with great timing which made it fun to play.
Saints Row 2 retained some good elements from the first, and a few new things that were a nice addition, but the multiplayer was a horrible down-grade. "Protect The Pimp" and "Blinged out Ride" were so fun in the original I don't understand why they removed them for the sequel. I can't even remember what the online game modes were called in SR2.
Going back to IV, as said above, it looks more like Saints Row 3.5 than a whole new game.