Hair types are nowhere near precision. They are just convenient because similar hair types usually respond similarly to styling procedures or products. Different hair types also respond similarly often so knowing the hair type can only take you so far.
My opinion:
3c, 4a, and 4b are less silky types. They types we usually call nappier.
3c
Kelis has this type of hair.
4a vs. 4b
4a has more defined coils and hangs more. I think that Aisha of this forum has 4a hair. Here's some pics:
http://motowngrl.8m.net/now.html
4b hair does not really hang unless there is a lot of it. Is more dense. Sometimes looks almost straight or wavy but not very coily. Here is a definite 4b:
http://www.picturetrail.com/dreamy7. I think this person is also mostly 4b:
http://public.fotki.com/N4U2LM/4_naptural_me_-/
den1, who was a feature of the month, also has 4b hair.
I think most people are a 4a/b mixture. I was not able to figure out my own hair type until I had several inches of growth, and I'm still not 100% sure simply because my styles involve a lot of manipulation in order to camoflauge the difference between my relaxed and natural hair.