4k@60Hz. The MacBook Pro 2015 with Intel Iris (I don't know about the Radeon GPU) only supports 4k@30Hz, which is terrible.
(My wife has the 1st gen MacBook 12", which has the same downside.)
Are you using the HDMI port? I'm using a 2015 with Iris and can run 4k@60Hz through the mini-DP.
https://support.apple.com/en-us/HT206587 shows that both 13" and 15" from 2015 should support 4k@60Hz.
The GPU must have a HDMI 2.0 output for 4k@60, and this is usually not present on all but the most recent laptops, producing the 30 Hz limitation.
On the other hand, DisplayPort 1.2 supports 4k@60. This is built into the Thunderbolt 2 ports on Macs, so as long as the GPU supports DP 1.2 it will work.
On my MBP (15"), the DP(Thunderbolt) + HDMI ports are wired directly to the Nvidia GPU. Parent mentioned his computer only had the Intel Iris graphics (13" model), which apparently can't do 4k@60 (so my previous comment was incorrect :( ).
I came across this while looking to see if I could buy a 4k display for my 12" Macbook (2nd gen.). My brief reading of it implied that it would be possible to drive it at 60hz. but maybe you are correct.
They are really cheap used or refurb