So I installed Windows 8.1 x64 using the Boot Camp Assistant on my Yosemite, iMac 5K 27".
The max screen resolution is being reported as 3840x2160.
I cannot explain what exactly is wrong, but my strain is huge after working in this setup for ~2 hours. It wasn't so in MacOS. I suppose the problem is with graphics driver.
I updated Windows 8.1 - everything I could.
What worries me is that the latest Boot Camp Support Software 5.1.5640 does not list iMac 27' 5K:
.. and the AMD website where I wanted to try their driver here: http://support.amd.com/en-us/download/desktop?os=Windows%208.1%20-%2064
This driver is not intended for use on AMD products running in Apple Bootcamp platforms. Users of these platforms should contact their manufacturer for driver support.
Then, I tried to contact Apple Support - I have an extended support for 3 years (AppleCare). They asked me nicely to go and ask Microsoft for this.
Finally, I contacted Microsoft Support and they proposed that I subscribe to their support service (e.g. spend another $100+) and they will solve my issue. They assured me they knew the solution and they even had an internal technical note about a similar case. Besides the feeling of being cheated, I supposed there should be drivers for my case, but I can't find anything.
Please help me - how else can I try to solve the issue, what system info you want to know in order to give advice, and where should I look for more references / reading?
UPD. I've also tried these drivers from AMD designed for BootCamp, but it did not help: http://support.amd.com/en-us/download/desktop/bootcamp