I have silky 4b hair sprinkled in all over my head. I will have to try to find a photo of my hair unmanipulated but I don't tend to allow my hair to be free like that.
My strands are fine and I have mostly cottony and silky texture along with (what some would consider) 4a and 4b strands. My hair straightens easily with tension (banding or braiding).
The letter/number combo refers to the curl/kink/wave size or lack thereof and the texture (silky/cottony/spongy) refers to the characteristics of the hair. Coarse/medium/fine refers to the diameter of each individual strand.
I'm not saying that b/c I don't think you know, I think that a lot of people get confused and think that 4b means coarse (synonymous with rough, in laymens terms) When that is not the case at all.
So like @
Tiye said, 4b can come in silky, cottony or sponge texture -- as can any other curl type (3a, 3b, 4a, 2b).