I have used a Philips 40" 4K (3840x2160) monitor via a RX480 GPU with great success for a very long time with Win 10 ver. 1703 and earlier.
In the windows display settings the monitors recommended settings also are the 3840x2160.
After updating to the Windows Fall update yesterday the resolution chanced down to some 2048x1080 making the use of the monitor impossible.
Spent yesterday evening Googling for advises and trying them out - but in the display settings there are no chose-able settings above 2048x1080 - which is also the Recommended setting.
Updating the RX480 card also changes nothing.
Rolled back the Windows "update" and now everything works again. Having the Windows Pro version on this PC I changed next Windows update date to Jan 14 2018 :)
Updated other PC having lower res monitors also - no problems there.
Just for your info.