Nigh impossible

What do you think of the other forms:
1) Nigh onto impossible
2) Nigh on impossible
3) Nigh to impossible
4) Nigh on to impossible

Which one(s) is/are correct? And do they all carry the same implication?

I would only note "nigh impossible", and perhaps "nigh to impossible". All others are too wordy.

Is #1 meant to be "Nigh UNTO impossible"? Otherwise, I don't see how it and the others make any sense.

I can't say anything either in regards to their correctness--they're all not technically incorrect. But they sound frempt.

"Nigh on" is the only variant I've ever heard.