Jesus' discourse on sowing and reaping:
35 Do you not say, ‘There are yet four months, then comes the harvest’? Look, I tell you, lift up your eyes, and see that the fields are white for harvest. 36 Already the one who reaps is receiving wages and gathering fruit for eternal life, so that sower and reaper may rejoice together. 37 For here the saying holds true, ‘One sows and another reaps.’ 38 I sent you to reap that for which you did not labor. Others have labored, and you have entered into their labor.”
(John 4:35–38, ESV)
Four months until harvest means it was December.
Say not ye? (Οὐχ ὑμεις λεγετε; [Ouch humeis legete?]). It is not possible to tell whether Jesus is alluding to a rural proverb of which nothing is known about there being four months from seedtime to harvest (a longer time than four months in fact) or whether he means that it was then actually four months to harvest. In the latter sense, since harvest began about the middle of April, it would be December when Jesus spoke.
Robertson, A. T. (1933). Word Pictures in the New Testament (Jn 4:35). Nashville, TN: Broadman Press.
When Jesus said "white unto harvest" he pointed to the Samaritans, who came to see Jesus because of the woman's testimony.
They went out of the town and were coming to him.
(John 4:30, ESV)
The writer and journalist H. V. Morton has a specially interesting suggestion about the ﬁelds white for the harvest. He himself was sitting at this very spot where Jacob’s well is. As he sat, he saw the people come out from the village and start to climb the hill. They came in little batches; and they were all wearing white robes, and the white robes stood out against the ground and the sky. It may well be that just at this moment the people started to ﬂock out to Jesus in response to the woman’s story. As they streamed out in their white robes across the ﬁelds, perhaps Jesus said: ‘Look at the ﬁelds! See them now! They are white to the harvest!’ The white-robed crowd was the harvest which he was eager to reap for God.
Barclay, W. (2001). The Gospel of John (Rev. and updated., Vol. 1, p. 195). Edinburgh: Saint Andrew Press.
Their clothing made the landscape white. At that point Jesus was harvesting.
They said to the woman, “It is no longer because of what you said that we believe, for we have heard for ourselves, and we know that this is indeed the Savior of the world.”
(John 4:42, ESV)
There were several sowers. The woman sowed the seeds Jesus gave her.
Come, see a man who told me all that I ever did. Can this be the Christ?”
(John 4:29, ESV)
4:36–38. In this context, Jesus and the Samaritan woman sow, and the disciples see the harvest (v. 39).
Keener, C. S. (1993). The IVP Bible background commentary: New Testament (Jn 4:36–38). Downers Grove, IL: InterVarsity Press.
There were the seeds from the prophets.
The woman said to him, “I know that Messiah is coming (he who is called Christ). When he comes, he will tell us all things.” (John 4:25–26)
The disciples did help prepare the soil by going into the town to buy food. This was a strange sight for the Samaritans.
For Jesus to lodge there, eating Samaritan food and teaching Samaritans (v. 40) would be roughly equivalent to defying segregation in the United States during the 1950s or apartheid in South Africa in the 1980s—shocking, extremely difficult, somewhat dangerous. The Jesus of the Gospels is more concerned with people than with custom.
Keener, C. S. (1993). The IVP Bible background commentary: New Testament (Jn 4:39–42). Downers Grove, IL: InterVarsity Press.