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.
6I have a big doubt about the honesty of this table. I tested two smartphones, one in BT3, the other in BT4 , with 3 cars (old> = 10 years). The BT3 works perfectly on 3 cars. The BT4 did not work on 3 cars. But it may be a problem due to Android ..– user350685Jul 28, 2014 at 7:10
1This is because the old cars are probably Bluetooth 3 single mode and it is not compatible with Bluetooth 4 single mode. You need to test Bluetooth 4 dual mode. Nov 2, 2014 at 17:55
Aren't pretty much all phones supposed to be dual mode though?– Steve MJul 26, 2015 at 19:13
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 exception are gadgets or peripherals based on the low-energy version called Bluetooth Smart. The protocol of Bluetooth Smart differs from that of older, so-called "Classic" Bluetooth devices. Bluetooth Smart devices are not backwards compatible and will not recognise nor pair with older devices supporting Classic Bluetooth. For example, an old Sony Ericsson phone sporting Bluetooth 3.0 will not be able to connect to a Bluetooth Smart device.
However, if a device supports Bluetooth 4.0, it can potentially recognise both Bluetooth Smart and Classic. If it does, it is officially labelled Bluetooth Smart Ready.
Personal health gadgets such as fitness bands and heart-rate monitors commonly employ Bluetooth Smart. These gadgets will only pair with smartphones or tablets that are Bluetooth Smart or Bluetooth Smart Ready.