P2P message Android

We are building an app where the core is the message exchange between up to 4 phones. All we need is this core code. A code that allows us to exchange messages in realtime, through all the connected devices. The major chalenge is that all the connected phones must receive messages from our server EXACTLY at the same time. It will possibly be required to test the latency between the devices and the server, in order to calculate the latency difference, so that it will be possible for them to see the messages exactly at the same time. As a test, we will run your code in 2 phones and check if both will see a test message from a server, at exactly the same time. Phones wont be connected to wifi, its all under 3G connection.