Because we had to find out the painful way: The Waveshare IO BASE Module B for the Raspberry Pi CM4 module does not work with the 4" DSI Touch Display from Waveshare - at least not as long as you use a BASE IO board revision < 4.

Only from board version 4 the higher performance DSI1 interface is used instead of the DSI0 interface of the Raspberry CM4 by the IO Base Board.