Clarification on 4k support for 60Hz Monitor I have an early 2015 Macbook pro retina 13' 

I read this article

https://support.apple.com/en-us/HT206587

It states that MBP will support

3840x2160 at 30 Hz refresh rate


So what happens If I try to use a 4K monitor which is 60Hz?
Monitors don't have their own refresh rates. They can SUPPORT higher refresh rates. So you always have to look at the weakest link between 1) what your computer can display, 2) what your cord can transfer, 3) what your monitor can display. For example, as is likely the case for you, if your monitor supports 4K at 60Hz and your cord supports 4K at 60Hz but your computer can only output 4K at 30Hz, then your monitor will only display 4K at 30Hz.

TL;DR: You'll only be able to see 4K at 30Hz.