Can I do 4K@60hz with my early 2015 MacBook Pro Retina?
I have a similar device (Early 2015 rMBP with 16 GB running macOS Sierra 10.12.6) and can confirm that it will power an external 4K display at 60 Hz - it's supposed to power two external displays at 4K (3840x2160) in addition to the builtin display, in fact. However, there are anomalies on my side when it comes to the refresh rate depending on my screen setup. Also, MacOS does not let me choose the refresh rate for the 4K screen when it's connected through the Mini DisplayPort to DisplayPort cable - other than feeling the difference, I only see the refresh rate in the system profiler.

Hardware:
rMBP Early 2015 with macOS 10.12.6 and 16 GB RAM;
Display 1 (old): 900x1440 via mDP-DVI adaptor and DVI cable; Display 2 (4K): 3840x2160 via mDP-DP cable

Scenario 1 = 1 desktop:
rMBP is closed,
Display 1 (old) is disconnected,
Display 2 (4K) is the only desktop displayed at 3840x2160 @ 60 Hz

Scenario 2 = 2 desktops with builtin screen: 
rMBP is open at 2560 x 1600 Retina,
Display 1 (old) is disconnected,
Display 2 (4K) is connected with 3840x2160 @ only 30 Hz

Scenario 3 = 2 desktops without builtin screen: 
rMBP is closed,
Display 1 (old) is connected with 900x1440 @ 60 Hz, 
Display 2 (4K) is connected with 3840x2160 @ only 30 Hz

Scenario 4 = 3 desktops: 
rMBP is open at 2560 x 1600 Retina,
Display 1 (old) is connected with 900x1440 @ 60 Hz,
Display 2 (4K) is connected with 3840x2160 @ 60 Hz

To sum up, this means that my early 2015 rMBP 13" will only display 4K @ 60 Hz on the screen capable of doing so if either all 3 displays are active or the 4K display is the only active display. The refresh rate on the 4K screen is dropped to 30 Hz if it's one of only two active displays. This result has persisted despite restarting my Mac but I do not know if this behavior is limited to my example. If any reader has a logical explanation and/or solution for this, please post your answer.