4TR control units are mostly with 3 connectors, however 4 pin version also exists. What is the most left first pin? I attach pics of two separate units, both have 4 connectors and the other unit has B+ on that and the other has A. The car I'm servicing has a cable connected to it and I have no idea what it is doing. Also the sticker, Slave 4.T.R., is confusing - what does it mean.