That processor has 4 cores. Hyperthreading lets it handle 8 threads at the same time. The hyperthreading hardware provides an extra set of registers for each core. Switching between register sets is extremely fast.
The net effect is only slightly slower than an 8 core non-hyperthreaded processor.
How many threads the program you are running can use was determined by the author of the program. If it was written as a single threaded program it will never use more than one thread on the processor.
If it was written to use 8 threads (highly unlikely) and you disable hyperthreading it would take twice as long to run.