Montage supports resolutions up to 1920x1080p, which is full HD. The device will work on 4k screens, but the image will be downscaled to match 1080p resolution. If a computer's screen resolution is higher than 1920x1080, the image will also be downscaled to 1080p.
The main reasons why Montage has not been designed to support 4K yet are that.
The bandwidth and network power required to stream your computer in 4k resolutions are very heavy. 4K image is much more demanding and puts much more pressure on your network; therefore, we chose not to support it at the moment.
Montage captures a computer's screen in full, meaning we transfer the exact amount of pixels. 4K computer screens are not that common yet, which reduces the practicality of offering 4K.