Where are you seeing a half-size M.2 slot available? The Alienware 17 R4 service manual available on support.dell.com indicates that this system has a 2.5" SATA bay and an M.2 slot that accepts a 2280 size SSD, which is the normal size.
Alienware 17 R4 Setup and Specifications
Storage Table 7. Storage specifications
SATA 6 Gbps•
PCIe up to 32 Gbps
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
search for "New Alienware 17 R4: Upgrade/Replace M.2 Solid State Drive" video on YouTube...
Look at video at 1:14.....shows it above larger m.2....
Interesting, the Service Manual doesn't explicitly mention that, but I can see on its diagrams that the slots are clearly shown. As for compatibility, I doubt you'll get an official answer from Dell about it since they of course can't test every possible SSD in this system, and unless they sell the SSD you're looking at from the factory, they probably haven't tested the one you're asking about. Still, the system specs you posted clearly indicate that the half-height slot supports both PCIe and SATA SSDs, and the unit you're looking at is the correct physical size for a half-height slot, so I don't see why it wouldn't work.
I bought a 64gb for $27.00 and installed it. Worked great.
If it did not I was just out $27.
Now in the future I have the option for 128, 256. or 512gb.
I can now wait for a low price...….
Thanks again for all the help.
Any 2242 M.2 should work in there as they are all half size M.2 form factor. The real kick in the nuts is that 512gb is seemingly as large as the 2242 size seems to go so it isn't super useful, but I guess it's better then nothing.