What does 1 Corinthians 14:4 mean?

Paul is showing how the spiritual gift of prophecy is better for the church than the spiritual gift of speaking in tongues or diverse languages. He has previously described the experience of speaking in tongues as speaking to God and not to other people. Perhaps this was the most common form of speaking in tongues, praying to and praising God in a language unknown to the speaker.

Paul now writes that speaking in tongues cannot build up the church as a whole. He says nobody knows what this person is saying. In the context of this passage, Paul seem to refer to how speaking in tongues was practiced in Corinth, rather than how it was meant to be done. It's likely the practice of speaking in tongues without an interpreter, and in a disorderly way, had become common in Corinth. Paul does not condemn the use of tongues for private prayer. In fact, Paul says that doing so would build up the one who prays by helping them see that God's Spirit was alive and active in this way. It would be encouraging, but only to that specific person.

Someone using the gift of prophecy before the church, however, builds up the entire church because everyone can understand those words.