UE4 and hyperthreading

First off, I know what HT is. What I am wondering is, will this boost performance, in say building lights and such, since, for example, a 4 core i7 will have 8 thread. Will UE4 build on each of those individual threads, or not?

By default it is disabled, but you could you enable it.
Take a look at this tutorial

Thanks! This really helps!

Now for another question. Would it be faster using the hyperthreading? Or just using the 4 physical core?

It depends which generation of HT you have in your CPU. Early generations (pre i3,i5 and i7) did not check to see if the the CPU already had one HT running, before giving it another one. Doubling up on one core is slower than running those two threads on separate cores. More modern CPUs DO check and run a thread on a core that isn’t being used (as long as one is available), which will give you faster performance.