Welcome to the Dell Community @wlkin 

Your Specifications say:

Interface:    SATA 6 Gbps (SSD Slot 1 to slot 3)
Interface:    SATA 3 Gbps (SSD Slot 4)

Hard drive:                   One 2.5-inch drive

Solid-state drive:         Four M.2 drives

Alienware 17 Specifications:

https://downloads.dell.com/manuals/all-products/esuprt_laptop/esuprt_alienware_laptops/alienware-17-r2_reference%20guide_en-us.pdf

Alienware 17 R2 Service Manual:

Best regards,

U2