I would like to add some more details about the 4K rendering so you can get more feedback.
The 4K is available within Downsampling for people which have most recent GPU and most recent driver (specially for AMD).
nVidia GPU have DSR (Dynamic Super Resolution), that allow the GPU to have a 4K rendering on any monitor. This features is only available on Kepler and Maxwell familly (GTX 5x0, GTX 6x0, GTX 7x0 and GTX 9x0).
For AMD user, this feature is called VSR (Virtual Super Resolution). However, it is available on certain recent GPU.
Back to the topic, I didn't try the nightly yet, but the targeting brackets work well for some minutes and then, it has moved on the upper left on the screen.
I will give you more feedback later this day.