Here in chapter four of John we have another case of “story sandwiching” similar to what we find frequently in Mark. As Jesus finishes his conversation with the woman at the well, she runs off to town to tell others about Jesus. The disciples show up and Jesus tells them that His food is to do the work that God sent Him to complete. He then tells the disciples to pray for workers for the harvest, people to spread and share the good news. At that point, the woman and people from the town arrive to check Jesus out and ask Him to stay with them.
People pray for workers for the harvest all the time. Often, what they are thinking of is for evangelists and missionaries to be sent out from churches to go to places where people need to hear. And, that is something that happens and needs to happen more. However, in this context I don’t think that is what John is talking about. The sandwiching of the teaching about the harvest shows that the workers Jesus is wanting his disciples to pray for come from the harvest itself.
Of course, in Jesus day there were no established churches. (Just Pharisees) So the workers had to come from the harvest. But things are not that different today. The best workers to reach people with testimony about Jesus are family members and friends. So pray for the harvest. And don’t just be the harvest, work in your fields.