Rega DAC users, Any info on Filter settings?

Even if you don't own the Rega DAC, does anyone know of a source of information that explains the different filter settings? The owners manual is pretty vague on this subject and I would like a little more definition if available. I have found that #4 seems to sound best on most recordings but would like to satisfy my curiousity.
Thank You.
Here's a bunch of info. There are a few links in some of the comments that provide even further options:
I use 4 too. I can't hear a lot of difference. It's a nice dac.