From the research I did it shouldn't be a problem. I went for 8GB and a 16BG Crucial. The memory is sitting in my lounge the iMac 5k should be here in the next couple of days. I can take the 2 sticks of 4GB out at somepoiint if I want to move up to 32BG. It saves around £230 on the apple price.
Alienware 17 R4 2017 review