The aRacer Speedtek 1 to 4 cable is necessary to add an autotuner, race module, or other modules to your aRacer ECU (except for the quickshifter). The quickshifter module does not use this port for connection so you do not need a splitter cable if you are only adding a quickshifter.
If you are only adding one module, such as autotuner, you can use a 1 to 2 Cable. Keep in mind that the iLink cable uses one port for programming, and the autotuner will use the second port. If you want to add an autotuner and race module, you will need the 1 to 4 Cable.