I don't think that my hair is 4b. My logic is this...the numbers represent the size of the coil/curl. My coils are the size of a pen spring/coffee stirrer (which is about 1/8" in diameter-see pic below). Many of the 4s I see have coils that are bigger than mine (about 1/4"). Add to that my coils do not clump together at all, and together these two things make a huge difference in care and styling.
Honestly, I would say that I am part of an unrepresented hair type, and I petition the addition of category 5. IMO, the difference is not between 4A and 4B, if your hair has a smaller coil, but it is the difference between 4 and 5.