So I know that one model maybe both of the 4s have Bluetooth built in and in theory it should work the way you want. All you should have to do is sync the 4 to the Sena as an intercom. But even though Sena is IMHO the best right now they still don't have their shit together so it may not work.
What does work is Sena has the GP10 GoPro Bluetooth Pack. This fits in to the back on the GoPro at the same place one of its Backpack Accessories goes. And then it uses a dongle to connect it to the camera via mini-usb and then you sync that as an intercom to your headset. That is my current set up. I am using a 3+ Black edition
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