They are not the same. The entire safety system of the A3 is different than the B4. The B4 carries on with some leftover bits from the B3 days, which is similar to the late A2. The A3 is different. Plus, all the B4s sold here came from Europe (Belgium, mostly), and used one seat belt supplier. All the A3s came from Mexico, with a choice of two different suppliers.
Now, this is not to say you could not bolt one in and make it work, however it may not be designed to work in the same way in a crash as the originals did. But I suspect there are other differences too.