If you are thinking of making a PC game that allows 4 phones to connect in, then...
It's possible, to have phone as gamepad, that connects to PC, etc. You could use bluetooth, TCP/UDP, or some networking protocol to send input information.
For Bluetooth, since HTML5 is basically a web page - it does NOT have permission to access bluetooth devices.
What about other things like TCP? In C2, We have WebSocket feature, and "WebSocket is a protocol providing full-duplex communications channels over a single TCP connection."
Is it feasible? From the C2 Manual:
ebSockets and multiplayer games
It may be tempting to use WebSockets to design real-time multiplayer games. Unfortunately, despite the fact they communicate in real-time, WebSockets are not currently a suitable choice for this. The underlying transport uses reliable transmission, meaning a single dropped packet can hold up all transmission until the packet is retransmitted successfully. For games with demanding real-time requirements, this can cause unplayable levels of latency. It is usually impossible to design around this without changing the transmission mode, which WebSockets do not support.
On the other hand, WebSockets should be suitable for games without such a demanding real-time requirement, like turn-based games. It should also be useful for application services, like chat rooms. Note this will still require you to create your own WebSocket server.
And one more thing to consider is:
Using the WebSocket plugin requires a WebSocket server. Construct 2 does not provide a server nor can the WebSocket plugin be used to make a server. If you don't already have a WebSocket server set up, you will need to create one yourself using a technology like node.js with WebSocket support. This can be a significant undertaking and require server-side programming knowledge.
That could be a bit overwhelming, and I am not saying this is the only method possible. Perhaps someone more knowledgeable in HTML5 and networking transmission might be able to address other possibilities of what you want more properly.
On a side note, you could also try some other engines, other than C2 running on mobile phone too, because C2 is strictly HTML5.