Hi there,
Today I bought a S4 and I feel the same with my Citroen C4.
Works perfectly with my S3 and S2 having my wife ... It's also very rare.
On the other hand, neither S2 nor S3 worked with Garmin GPS (Nuvi 1490) and the S4 works perfect, so if you can not find how to fix it, the solution will be to use the GPS as a hands free for me, and the stereo handsfree car as my wife (who stayed with my S3)
I read in other forums using the BluetoothKeepalive (which is paid) or Bluetooth Service (free) but still did not go to the car to try (I have not much faith you ...)
I do not know if it's a firmware problem (my computer is a S4 active ATT USA) or are a subject of BT.
I hope to find the solution. (and will post here)
regards