Hi Carsten,
You cannot get two 4K displays from a single DP port. The MST Hub split the bandwidth on 4 ports and the maximum on each does not reach 5 Gbps or 1080p. A DP port has a maximum bandwidth of 21.6 Gbps, a 4K signal reaches almost this bandwidth. 
To get two 4K displays running you require to have two individual DP ports from a graphics card or a supported device.