Only some processors are HyperThreaded;  only these will benefit from using twice as many threads as you have cores.  The hyperthreads will only run at 2/3 to 3/4 the speed of the primary cores, so you won't double the render speed, but the gain is significant, about 167%-175%.

     One of my machines has a Core2 Quad core (not HyperThreaded);  it renders best at four threads.  Another machine has a core i7 quad (which is HyperThreaded);  it renders noticeably faster if set to eight threads.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5