If i were in your shoes, i would disconnect all drives except the 4 TB drive, then boot into the Solus live usb, launch the program Gparted to change your 4TB disk to GPT.
You do that under "Device" > "Create Partition Table", in there you choose "GPT" (Be aware that you will loose all data on the drive, so backup any data you dont want to loose), while you are in there do yourself a favor and create the boot partition, using the guide from the Solus Help Center: https://getsol.us/articles/installation/disks/en/
"If you are using a system with UEFI, you may need to create a EFI System Partition, also referred to as an ESP. This is not necessary if you are enabling Solus to install onto the entire disk.
To create an EFI System Partition, open up GParted and create a FAT32 partition that is 512MB in size. Next, right-click on the partition and click Manage Flags. On the Manage Flags section, enable the boot and esp flags.
Notes:
Your system must be booted using Unified EFI (UEFI) mode, as opposed to a “legacy (BIOS) mode”.
Secure Boot needs to be disabled.
Your disk is required to be GPT formatted.
If you cannot see your SSD drive, set the SATA configuration to AHCI."
Once you have done that, reboot your system and boot the live usb again, before trying to install Solus.
Be sure that you have read:
"Preparing to install"
https://getsol.us/articles/installation/preparing-to-install/en/
"Disks"
https://getsol.us/articles/installation/disks/en/
If the installation completes without issues, you can connect the other drives again. To boot from your 4 TB harddrive with Solus, you need to activate your bios boot menu, on my motherboard it is shown by pressing F11 (MSI is F11, i think Asus is F8, you need to spam it on boot up), choose the 4 TB drive and you should boot into Solus.
All i have written about, is assuming that your bios is set to EFI (EUFI) mode, not "legacy (BIOS) mode", and that your SATA configuration is set to AHCI.
Note that if it were in IDE mode and then changed to AHCI, your Win 7 installation may not boot, then you need to search how to switch Win 7 over from using IDE to AHCI mode, without having to reinstall (i have done that before when i used Win 7) Let me know if i need me to find a guide for you, if you run into that problem.