BoA 4.3 is the current version of BoA. It is available since June 30th, 2012.
An important difference between BoA 4.2 and BoA 4.3 is the management of color attribute of the objects. Until BoA 4.2 the color of the objects was selected among a choice in a 256 colors pallet. Since BoA 4.3 there will be 16 million colors available.
Because of this change, the BoA files will have a storage format a little bit different in BoA 4.3. Of course BoA 4.3 will be able to read files written with BoA 4.2, without any handling, and rewrite them with the BoA 4.3 format without any action from the user. The files written by the older versions of BoA (including BoA 1.0) will be also perfectly readable by BoA 4.3.
In the other way, BoA 4.2 will continue to be able to read all the BoA files, including those written by BoA 4.3. However in this situation, the color of the objects will not be completely recognized. A conversion of the colors towards the 256 colors pallet will be processed.