First make sure your bios isn't disabling any cores, then run msconfig in windows, select the boot tab, then advanced and make sure "number of processors" is correct. Also check Windows task manager and see if it reports 4 cores 8 threads (which it should)
If these are all correct then what's likely happening is windows is identifying the FX 8300 as a hyperthreaded quad core which is normal for FX 8xxx CPUs as although they technically have 8 integer cores those cores are situated in 4 modules which each have one FPU between the two integer cores. Windows does this to avoid overloading the FPUs
