The disciples came back with some food and wanted Jesus to eat something (likely for nourishing). But Jesus says he has food (4:32). At first, the disciples were a bit confused and thought someone else got him food.
Once again, Jesus is doing the figurative instead of literal. He stated that the "food" is "to finish his work." (4:34) Contrary to the disciples' expectations, the "harvest" is actually ready for them (4:35)! Based on the context and how some of the work is "gather[ing] fruit for eternal life" (4:36), the work that Jesus is referring to is sharing Jesus's gift of forgiveness and renewal. In essence, Jesus proclaims that there are a lot of people that we can share Jesus with -- as of today.
Sharing Jesus is a team thing, especially since Jesus proclaims on how different people are sowing, reaping, and laboring (4:37-38). Nevertheless, even with the different people on the same team, Jesus still calls the disciples (and in extension, us) to play our role and jump in. Additionally, because of how reaping leads to "receiv[ing] wages" and "gather[ing] fruit for eternal life", the reapers and sowers can "rejoice together" (4:36). It's one thing to personally accept Jesus's gift of forgiveness, but it's another thing to see all your friends also accept Jesus's gift of forgiveness (since they can also experience renewal).
Even today, Jesus is calling us to share him. Conveniently, there is an example conversation that Jesus did just a few verses before that we can look to for reference. So we can go and tell our good buddies who Jesus is.