1. Unfortunately I have no information about amount of BT-4 produced. I'm sure only that they did take part in WW2
If I obtain such information I certainly will publish it.
2. It was 37mm paratrooper's gun. Its burrel and ballistics were like 37mm Anti-aircraft gun M1939. A special subcaliber (not sure about correctness of the term translation) armor-piercing shell was designed for it.