The lower one, K4a, is very shaly, K4b is mainly halite, and K4c is mostly anhydritic, with halite only as thin beds.
This unit clearly is more homogeneous than the precedent K4b and can be easily differentiated, both on well logs and outcrop.
In the medium part, K4b, the evaporitic sequences are fully developed and reach the halite stage, maximum brine concentration of the K4-K6 sequence.