Macbook and 4K monitor
Answer is yes with the right USB C to DP adapter. I have a 2015 MacBook and I have tried it with two very different 4K displays - one Philips, one LG - and both worked correctly at 4K 60Hz. If fact, they work in Bootcamp, too at 4K 60hz. Impressive. 

FYI, I understand this capability arrived with the Sierra MacOS update. Earlier MacOS versions limit you to 30Hz on this MacBook.

I use a Club3D branded adapter.