I don't think you can say that stiffer is "better". It's different. I would recommend getting a spanner wrench so you can adjust it properly. Don't use the old screwdriver and a hammer method that every previous owner of my bikes has.

Go out and pick a circuit of roads that is typical of the type of riding you like to do. Start at the stock setting and take a few laps. Then adjust the spring preload up by moving it from setting 2 to setting 4. Take it out and see how it feels. If you're like me, you probably won't be able to feel a big difference. Then bump it up to 6 or 8 and ride it again. It's all about feel. Whatever feels "better" to you is what you should ride. Here is a link to a general understanding of bike suspension settings and what effect they have with adjustments. Linky: http://www.sportrider.com/tech/146_0006_susp/

 

on the f4i it's kind of hard to use the hammer and screw driver anyway.