A8-5600K is built on the Bulldozer architecture. It contains 2 modules. Each module contains 2 integer execution units and 1 shared floating point unit. Thus, there are a total of 4 integer execution units and 2 floating point units. You have 4 cores that can work on 4 threads.
Prior architectures have a 1:1 ratio of execution units to floating point units, so some software detects only 2 floating point units, and declares that is a 2 core processor.