What does John 9:4 mean?

Jesus has just clarified a common misconception held by those who hold to the idea of karma. This idea suggests that a person's suffering is always, somehow, brought about by their own actions, even those of a former life. In the disciples' case, their culture believed that all mishaps were divine punishment: either the man, or his parents, must have been guilty for the man to be born blind. Jesus refuted this in the prior verse, clearly indicating that this blindness was in no way caused by the man's own sin, or that of his parents.

Here, Jesus uses metaphors which relate to His earthly ministry. In an era without electricity, nighttime was, by necessity, a time where less work could be done on account of darkness. The day was the most effective time to work, but it is also a limited time. As used by Christ here, "day" refers to the time left for Jesus to serve God's will on earth. "Night" refers to the rapidly-approaching time when Jesus will die, ending His earthly work.

That parallel makes Jesus' use of the term "we" very important. Jesus knows He has a finite amount of time to "work" in His earthly ministry. Likewise, we as Christian believers have only the days of our earthly lives to accomplish the work God has assigned us in that time. Rather than simply saying, "I must work…" Jesus places an obligation to be responsible with our time by saying "we must" do so—meaning all who serve God.