I have 15 huacayas & 1 suri.
4" is a good staple length for a year's growth. Younger alpacas will have longer staple lengths, since they're just growing not breeding, gestating & nursing. Adult alpacas put more energy into reproduction than fiber (fineness, crimp & staple length), which can result in fiber that is less fine and shorter as the alpaca gets older. However, there are exceptions to this. Most mills require a staple length of at least 2".
Darker colors don't necessarily mean coarser fiber. Fineness is more a function of genetics than color. Our finest animals are a brown & a black, both from bloodlines known for extreme fineness.
The structure of the suri fiber is different from the huacaya, which makes it more slippery & shiny. We have a huacaya girl who's fiber is very difficult to separate, which is due to her fiber structure. Top quality huacaya will exhibit bundling, in which the fiber organizes into locks, somewhat like the suri.
There's a section on this website with articles about alpaca fiber. http://www.alpacas.com/AlpacaLibrary/