An E4 error indicates that there is a communication error between the control panel and the steam generator. In the first instance, reset the shower and see if it clears the issue. By resetting we mean pressing the reset button on the RCD Plug (as below) not turn the control panel on and off within the cabin.
If resetting does not fix the problem, check the connections between the control panel and the steam generator are correctly clipped in and aligned correctly.
If all the above fail to fix the issue, then it's likely the steam generator is faulty (this houses all the electrics which communicate between the control panel and steam generator) and will need replacing. If your product is under warranty, please submit a support ticket along with your warranty number to obtain a replacement. If outside warranty, please visit our spares section where you can locate a replacement.