Connecting external display to Early 2015 MacBook Pro
Display Support:

Supports an HDMI-compatible device, including 4K, while using one Thunderbolt display or support for two Thunderbolt displays

External Resolution:

HDMI


3840 by 2160 pixels at 30Hz
4096 by 2160 pixels at 24Hz


Thunderbolt


3840 by 2160 pixels at 60Hz
5120 by 2880 pixels at 60Hz (with Radeon R9 M370X)