How many cores of CPU can UBT use?

I am planning to buy a new PC, and I am stuck between Intel i7 5820k, and 4790k. Compiling UE4 takes a lot of time, so I’d prefer one which can reduce more compile time because I am modifying the source code of the engine. If UBT can use more than four cores, I guess 5820K would be a better choice. Otherwise, 4790K probably will be better due to more processing power each core. Do anyone have any suggestion?