dpi (dots per inch) is a measurement of camera resolution, not of the speed of the sensor processing. So the full processing goes like this:

The optical sensor in the mouse takes an image at 4000dpi resolution 60 times per second (60 Hz). That information is then modulated onto a radio wave that swings up and down 2. 4 billion times per second (2.4 GHz) - but the information is still only sent 60 times per second, so less often than would be of a modern gaming screen updating 144 times per second.