What makes you think there is a 4TB limit in the first place? BIOS-based systems have a 2TB limit for partitions because they use the MBR partitioning scheme. If you are using UEFI, then the boot drive can be larger using GPT partitioning (which is presumably the case if the PC currently has a 4TB boot drive). A BIOS-based system can also have an MBR drive less than 2TB as the boot drive, and then you can use GPT for a storage drive that is 12TB. You seem to have UEFI anyway you should have no problems at all and can just connect the drives, set up the RAID, and configure the drive as GPT, or you can eliminate the 4TB drive and make the RAID array the boot drive using GPT. (Installation of OMV will presumably format it properly.)
I don't know what any of that has to do with PCI, though.
WaterPanther incidentally doesn't appear to be particularly good. They're just refurbing other brands from what I can see, or reselling rejects from them.