MaxArk :
Its a GigaByte Z170XP-SLI-CF(U3E1)
Ok - that supports UEFI boot for sure.
You have several options.
Option #1. Clean your 4TB drive, convert to GPT, reinstall windows (but I'm guessing you'd prefer not to go this route)
Option #2. Get another empty drive with size at least 500GB
a) clone OS drive to 500GB drive, change boot priority in BIOS and boot from 500GB drive (still legacy/MBR boot);
b) then clean 4TB drive, convert to GPT;
c) create EFI system partition on 4TB drive;
d) clone OS partition from 500GB drive to 4TB drive (partition to partition clone mode);
e) fix bootloader on 4TB drive using bcdboot;
f) change BIOS boot priority back to 4TB drive and boot mode to UEFI and boot from 4TB drive.
Done (reinstall avoided).
Does that sound like fun thing to do?
