I have been unable to find an answer for this. The last spec I seem to find that mentions backward compatibility is 2.1.
They are backward compatible. But - since each version brings new functionalities with it, only the basic function set is backward compatible all the way, see the table for more detail. In addition to that 2 devices wanting to communicate using bluetooth have to support the same profile(s), the full piece that i copy/pasted from here.
Not if it is Smart
In general, Bluetooth is backwards compatible: Bluetooth devices supporting the just-announced Bluetooth 4.2 standard should still be able to pair with devices using, say, the ancient Bluetooth 2.1, launched back in 2007.
The exceptions are gadgets that use a low-energy version called Bluetooth Smart, which works on a different protocol than older, or "Classic" Bluetooth devices. Bluetooth Smart devices are not backward compatible and won't recognize (or pair with) older devices that support Classic Bluetooth. (For example, an old Sony Ericsson phone sporting Bluetooth 3.0 won't be able to connect to a Bluetooth Smart device.)
However, if a device supports Bluetooth 4.0, it can potentially recognize both Bluetooth Smart and Classic. If it does, it's officially labelled Bluetooth Bluetooth Smart Ready.
Gadgets that commonly use Bluetooth Smart include personal health gadgets such as fitness bands or heart-rate monitors. These gadgets will only pair with a smartphone or tablet that also uses Bluetooth Smart or that is Bluetooth Smart Ready.
protected by Community♦ Oct 8 '14 at 20:47
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