-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.