Each additional thread splits the rendering of the image. You always want the number of threads to equal to or less than the number of actual CPU cores (or execution threads) you have available on your system. Never use more threads than the number of cores/cpu threads you have. The more threads you are able to use, the faster your renders will be, generally speaking. Most systems these days are dual core or quad core, so you can use 2-4 threads, depending on the system. Your X4 should I think have 4 cores.
- Oshyan