I use a 4K Alienware laptop every day with two Full HD monitors attached to it through the official Alienware dock. I'm using Windows 10 and due to internal company policies, I'm not using the anniversary edition which supposedly brought better environment for multi high DPI monitors support.

I can say that it is pretty usable, but whenever moving windows between monitors you have to fully move them otherwise you will get weird results. Another caveat is that not all apps support 4k so I get really small icons in some applications such as greenshot.

All in all, it's decent and one can live with it.