HD Graphics via Intel HD 4600 and NVIDIA 850M.
The tables on pgs 18 & 19 would seem to indicate that you can only connect one 4K external display using DP but again, I'm not using the internal display so wasn't sure if that freed up any bandwidth for a second display. If you scroll to page 13 you can see an example of a DP 1.2 hub making use of the MST to support multiple monitors from one DP source but there is no mention of display resolution or refresh rate with that visual. I found this on the VESA website which is what prompted the question: I am not using the internal display on the laptop when the two external monitors are in use so I'm not sure if that means the bandwidth allocated to the eDP is then available for external DP connections.
I contacted Alienware/Dell tech support before posting this and their reply was no it can't because it's only on DP connector which to me meant there was a lack of understanding that the DP spec allows for more that one display to hang off of a DP output.