How do I connect 4 or more Arduinos to a single shared GSM Module?
You should consider connecting one Arduino to the GSM module, and connect the 4 Arduinos together using SPI, I2C, or any other inter-hardware comms protocol. Before I knew these options existed, I once wrote a bit of code to send data between Arduinos by having one pin represent the data being sent and another pin changing every time a new bit was available. You could do something similar with an extra line per pairing, so an Arduino could tell the primary that it had new data to send.
Anyway, check out the &quot;Wire Library&quot; - it's basically for this kind of work. I think you'll be happier trying to have one Arduino &quot;in charge&quot; and connected to the CSM while the rest vie for its attention.