Can you define the hook up scenario? I assume the 4101 is connected to PSTN lines and is being used with the 4004 to extend the PSTN lines to the 4004 which has analog phones connected to it, correct?
While I could be wrong and you may want/should to contact GS support for better guidance, you should refer to the GXW4004 manual and look at enabling call features and then the dial codes. If my understanding of the scenario is correct, you are only extending the PSTN network such that analog phones on the 4004 can dial out the respective PSTN line on the other end at the 4101. So, when you hook flash and then enter a number (as you have stated), my guess is that you are essentially trying to get the FXO on other side to accommodate another call on the PSTN such as conducting a 3-way call and, this assumes
the provider is supporting the function.