What does 2 Corinthians 2:4 mean?

Many Bible scholars believe that what's described here is a now-lost letter of Paul. This would have been sent between what we now know as 1 Corinthians and 2 Corinthians. Some suggest the mystery letter was actually written prior to 1 Corinthians, but that's not as clean a fit with the context of those two letters.

Instead, the sequence seems to be that Paul visited the Corinthians briefly following the writing of 1 Corinthians. During that painful visit, a confrontation took place. This appears to have been with a church member challenging Paul's authority as an apostle. It is unclear if most in the church sided with Paul or with this other man. Paul wrote the now-lost letter to them in order to urge the church to deal with this man and make clear their support for Paul's authority as an apostle of Jesus.

Writing that letter, Paul now says, caused him great pain. He wrote it out of affliction and anguish of heart, shedding tears as he sent it. He did not write it in order to cause them pain. That's the last thing Paul wanted for them. Instead, he wrote it because he loved them so deeply. He knew that hearing the truth might hurt them, but he also hoped it would bring healing to everyone involved. Apparently, that's exactly what has happened.