It's also possible to shoot cropped 2K while in 4K readout mode by setting the Height to 1088px and manually cropping out the center of the image. In this configuration the maximum frame rate is 833fps. The image quality will be better than subsampled 2K, but obviously not as good as full 4K. See below for an example of 4K, cropped 2K, and subsampled 2K.