Does the system disk occupies one of the 6 slots?—If you don’t install system on slot 4, then it can be used as storage .
Could Gen4 SSDs also be used?— Yes, but the speed is still PCIe3.0*2
Do they also have to be x1 lanes? Does the device support x4 lanes too?—Slot 4 supports PCIe 3.0 ×2 (for SATA3 HDDs and SSDs), while the other 5 slots are PCIe 3.0 ×1.No x4 lanes are supported by the device. The specs explicitly list PCIe 3.0 ×1 and ×2 as the maximum lane configurations
Presuming all 6 drive bays are installed with the same spec SSDs, what RAID options are supported?– It supports soft Raid