2 X 4 cores = 8 cores. Then i decided to enable Hyper Threading on the server. Guess what! now i got 16 cores after enable Hyper Threading. Well, not a big deal. That's what HT is for: it double up your CPU cores.
The next question that pop out from my head is
"Will Hyper Threading affect Hyper V performance?"
After surfing for a while on this question, i managed to find a page which discuss about this question, please refer to http://www.windowsitpro.com/article/virtualization/q-does-hyper-threading-affect-hyper-v-.aspx
As a summary, Hyper Threading will never hurt performance but aid performance. So just go to BIOS and enable Hyper Threading to enjoy extra cores and performance.
FYI, Hyper-V R2 now supports up to 64 logical processors (cores or hyper-threaded CPU's).