http://topics-cdn.dell.com/pdf/alienware-17-laptop_Setup%20Guide_en-us.pdf

Hard drive
One 2.5-inch drive
Solid-state drive (SSD)
Two full-size M.2 PCIe/SATA drives
One half-size M.2 PCIe/SATA drive

You can replace the 2.5 with an SSD

HOWEVER I am not aware of any 4TB SSD's in either the M.2  physical size that is TB

On the 2.5 inch models PM1633a SSD is quite fat.

Samsung 4TB 850 Evo 2.5" SATA III SSD might fit but its $1600

Installing drives larger than 2TB is not trivial.  Average user will not know how to install the proper AHCI F6 driver with WINPE and use diskpart to format and mount the drive then finish the install. It must be converted to GPT mode before it can be formatted as a single partition.

shift f10 to open adminstrator command prompt

then run diskpart

WD Caviar Green 3TB Hard Drive Installation Demo - YouTube 

SSD 850 EVO 2.5" SATA III 4TB Memory & Storage - MZ-75E4T0B/AM | Samsung US