Why Moto X Play 21MP Camera can't shoot 4K videos?

To begin with, the megapixel count alone cannot be an assurance for a better image or video capture quality on your smartphone. Much depends upon the sensor, the pixel size and the associated software. You cannot always have good picture quality on a 21 MP camera than that on a 13 MP camera. A classic example could be iPhone. Unlike Android devices which boast off by making hue and cry about special features, an iPhone does not have those " extraordinary" extras, yet the camera quality is always better on it.

Coming to your question on the lack of 4K video shooting on Moto X Play, the reason for the same hinges on what I have stated above. The Moto X Play runs on Qualcomm Snapdragon 615 processor. The processor does not support 4K video recording. I remember having read about the lack of it on a Qualcomm article.

4K is a part of Ultra High definition horizontal resolution. It is based on the horizontal resolution that renders 4000 Pixels at a time. So just lens with higher megapixel count doesn't help. In order for it to capture 4K input the hardware has to support the same.

UHD input from the phone wiht smaller screen resolution would not be possible. Though your query is reasonable for those phones not able to support 1080p 60FPS images, which i theory they should unless they lack image sensors. The current UHD output requires support for a better processor that processes those graphical rendering.

Moto X Play doesn't seem to be supporting 4K image sensor and the processor that renders the specified UHD input. The same can be said for the OnePlus One device.