Connecting a 4k monitor to my mac book pro

2nd Display Support: Dual/Mirroring* 2nd Max. Resolution: 3840x2160 (x2*)

Details: *This model supports a simultaneous maximum resolution up to 3840x2160 on two external displays via Thunderbolt 2.

Alternately, it can support a single display up to 3840x2160 via Thunderbolt 2 and

a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.

from:

https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i5-2.7-13-early-2015-retina-display-specs.html

if you use plug an HDMI cable into the built-in HDMI port, you can get up to 30 Hz at nominal "4K" resolution.

Alternative connections using adapters may be possible. You did not post the make&model of the display, so readers can not research alternative connections for you.

Hi,

If you connect your external monitor via HDMI,

• HDMI video output
• Support for 1080p resolution at up to 60Hz 
• Support for 3840-by-2160 resolution at 30Hz
• Support for 4096-by-2160 resolution at 24Hz

And connect via Thunderbolt port, you can connect resolution 4K of 3840 by 2160 at 60Hz or even 8K at 60Hz.

This MacBook Pro is a 2015 model.

To support displays WIDER than 4096 pixels wide, this model requires use of TWO cables to connect to the display.

Wider than 4096 pixels wide on one cable requires the 7th generation Kaby Lake processor or later, first shipped in the 2017 models.