4b and 4c hair types look very similar and are similar, the main difference is 4c hair has tighter coils. This makes 4c hair more delicate and there is little to no visible curl definition unless the hair is manipulated.
The care routines and the types of products used are virtually the same, the only differentiation is when it comes to care.
For more hair growth, generally speaking 4c hair will thrive wearing protective styles more often than 4b hair type. This is especially true if you have 4c hair with strands that are thin or fine.