8 logical threads at 4 cores will at a maximum run 4 times faster in parallel?

This is a quick summary of Hyperthreading

Thread switching is slow, having to stop execution, copy a bunch of values into memory, copy a bunch of values out of memory into the CPU, then start things going again with the new thread.

This is where your 4 virtual cores come in. You have 4 cores, that is it, but what hyperthreading allows the CPU to do is have 2 threads on a single core.

Only 1 thread can execute at a time, however when 1 thread needs to stop to do a memory access, disk access or anything else that is going to take some time, it can switch in the other thread and run it for a bit. On old processors, they basically had a bit of a sleep in this time.

So your quad core has 4 cores, which can do 1 thing at a time each, but can have a 2nd job on standby as soon as they need to wait on another part of the computer.

If your task has a lot of memory usage and a lot of CPU usage, you should see a slight decrease in total execution time, but if you are almost entirely CPU bound you will be better off sticking with just 4 threads

A i7-2670QM processor has 4 cores. But it can run 8 threads in parallel. This means that it only has 4 processing units (Cores) but has support in hardware to run 8 threads in parallel. This means that a maximum of four jobs run in on the Cores, if one of the jobs stall due to for example memory access another thread can very fast start executing on the free Core with very little penalty. Read more on Hyper threading. In Reality there are few scenarios where hyper threading gives a large performance gain. More modern processors handle hyper threading better than older processors.

Your benchmark showed that it was CPU bound, i.e. There was little stalls in the pipeline that would have given Hyper Threading an advantage. 50% CPU is correct has the 4 cores are working and the 4 extra are not doing anything. Turn of hyper threading in the BIOS and you will see 100% CPU.