1.SB3 = The original game. You should fap to this one first until you get tired of it.
2.SB3 Plus = The original one with new sex positions and secret characters.
3.SB3 Gravure = It's a quick spinoff of SB. It lets you take pictures of the girls in 40 different erotic positions.
4.SB4 Flash = Shitty game. I never really discovered what Flash was about. It's meh.
Note that the other 3 games wont work until you run the registry file.
Edit it until it has the full path of the games. It should be something like this:
Windows Registry Editor Version 5.00