Neil Young recorded the song for his 1978 album Comes a Time, with harmony vocals from Nicolette Larson, and on The Band's The Last Waltz. It has received significant airplay over album oriented rock and classic rock radio stations and has become part of Young's concert repertoire, including featured performances during Young's yearly appearances at Farm Aid benefit concerts.

Based on this passage, who sang four strong winds with neil young? Let's give stream of consciousness first:
To answer the question, consider the following: Neil Young recorded the song for his 1978 album Comes a Time, with harmony vocals from Nicolette Larson, and on The Band's The Last Waltz.
So, the final answer is Nicolette Larson.