Were you previously able to put out 4K resolution onto an external monitor, from your Mac? But, are you not able to do that after upgrading to a recent version of Sierra?
How to fix Sierra’s inability to display 4K on an external monitor?
With Sierra’s new update, it is designed in such a way that 4K can be cast onto an external screen only via the HDMI port and not the mini-Display Port that you might have used previously.
This is particularly true for MacBooks and other Macs that were bought before 2014. If you have a more recent Mac, there is a chance that you can make 4K work with just your mini Display Port.