You may need a Thunderbolt dock for that -- dual 4K isn't one of the supported configurations listed. [URL]  
Hello, I have the DELL-WD19S130W connected with two 4k monitors. Does the laptop (xps-15-9530) support both 4k (60Hz) displays and charging via the dock? Based on the spec (charing with 130W and Thunderbold 4) I think it should work. Can anyone confirm? Thanks, Jonah
Dell XPS 15-9530 and DELL-WD19S130W