This workaround does require you to reboot, but does not require to change settings in the BIOS/UEFI.

The idea is to add a boot entry to the Windows Boot Manager [1] which limits the system to only four cores [2].

First, make a new boot entry by copying the current one. Start command prompt as administrator (right-click and select "run as administrator"). Type in the following and press enter: bcdedit /copy "{current}" /d "Whatever name you want"

Next, start sysconfig and select the entry you just created. It will be listed under the 'boot' tab. Once selected, click on "Advanced options..." and check "Number of processors" and change it to 4. Click "OK". Change the timeout to something you think is acceptable (three seconds is good) and click "OK".

When you reboot, you see a screen where you can select which entry to boot. Just use the arrow keys to select the entry you created.

Hope this helps.

PS: If you use GRUB (or some other boot loader) before the Windows Boot Manager, you seem to need to select the entry in GRUB twice: The first to start the Windows Boot Manager as normal, and then after you select a non-default entry in the Windows Boot Manager, you will be returned to GRUB and must select the Windows Boot Manager again. Making GRUB default to booting the last entry that booted [3] is something you can consider. Or it might be possible to have GRUB handle the entire process, but I don't know how you'd go about doing that.

[1] docs{dot}microsoft{dot}com/en-us/windows-hardware/drivers/devtest/adding-boot-entries
[2] answers{dot}microsoft{dot}com/en-us/windows/forum/windows_7-gaming/timeshift-crashes-on-pcs-with-more-than-fou r-cpu/d8811db1-0736-4516-a 7f8-5b0d030ab2b6
[3] www{dot}maketecheasier{dot}com/set-grub-remember-last-selection/