MBR doesn't do 8Tb, it maxes out at 2Tb, so if that was a single drive with partitions it'd automatically have to be GPT.
A brand new drive should show up as non-partitioned single unit. Any partitions being user added.
A single drive with partitions is easily cleared in diskpart, a few commands and done as there will be only one drive listed, with multiple volumes, but multiple physical drives won't list the same way
From everything read so far, it seriously points to a fake where lesser cards have been substituted. Whether they are in fact 4x 2Gb drives slapped in a single enclosure or 4x smaller drives is in question.
You'd have to open the cover and take a look, visually verify the drives being single or multiple, get serial/model numbers etc.