Yes I can confirm that I have connected a 4K monitor to my Zbook 15 G3 using a USB-C (thunderbolt) to DP adapter and also using a DP to HDMI adapter. The dock has both USB-C and DP ports on it. I have not driven 2 x 4Ks at the same time since I only own 1, but even with the Intel 520 video you should have no problem. Mine drives a 4K and a FHD at the same time with no problems at all. I am told that each thunderbolt port can drive 4 x 4K monitors. I would suggest connecting the 2 4K monitors to 2 different ports.
HP has loaned me the Zbook 15 G3 mobile work station in connection with my volunteer work on the Forum.