Using RAM Slots 1 & 3..........

How do you know slot 4 (B2 I presume) is faulty?

Putting that aside, if B2 slot is broken in such a way as to not interfere with other slot operation, then I'd suggest using A1 and B1 (slot 1 & slot 3 in your lingo).

The electrical termination characteristic won't be ideal, but if it works, well, it's better than nothing. If this doesn't work, I'd try reducing the memory speed by a notch.

I would not use B1 and A2 (slot 2 & slot 3 in your lingo) because that introduces asymmetry, even though you're still using both channels. It probably doesn't hurt anything to try it though, if A1+B1 doesn't work.

If all of that doesn't work, well, you'd be left with using A1+A2, which means, your total memory bandwidth will be cut in half. But well, desperate times call for desperate measures. Hopefully, you won't have to resort to that.