-Inversion-
1. Down came the rain.
2. Down it came.
3. Down came that.
4. Down that came.
(I think #1 and #2 are in right order. What about #3 and #4? Which one is correct, #3 or #4?)
I would not use 3. I would rarely use 4, but not when speaking about rain.
1 and 2 are fine.