Hello and welcome.
Graphics Card Intel Iris Pro 5200 (2.2 GHz) or NVIDIA GeForce GT 750M and Intel Iris Pro 5200 (2.5 and 2.8 GHz)
Graphics Memory 2 GB GDDR5 (GeForce GT) Up to 1.5 GB shared from main memory (Iris Pro)
Display Connection 1 - HDMI port or Thunderbolt digital video output
Display Support Supports an HDMI-compatible device, including 4K, while using one Thunderbolt display or support for two Thunderbolt displays
Display Modes Dual display extended, video mirroring, and AirPlay Mirroring
External Resolution Up to 2560 by 1600 pixels (Thunderbolt) Up to 3840 by 2160 pixels at 30Hz or 4096 by 2160 pixels at 24Hz (HDMI)
Looks like only a single 4K display, regardless of which model 13" or 15" and which graphics card.
C