What does John 13:4 mean?

The disciples see Jesus as a lord and master. When Jesus washes their feet, the act is shockingly humble. Ancient roads were covered in dirt, dust, waste, and animal droppings. Ancient people wore sandals or went barefoot. Washing of feet was necessary, but dirty and inconvenient. This is why Jesus is obligated to change clothes—this was a messy job.

In that era, people would most often wash their own feet. When done for hospitality, it was always performed on someone of higher status by someone of lower status. The task Jesus completes is, by definition, one performed as an act of servitude and modesty. By the standards of that era, it was ludicrous for a master to wash the feet of their inferiors. This is exactly the point Jesus seeks to make. Once the task is done, He will explain how His example shatters our natural pride and vanity. In no sense does Jesus deny that He is the Master (John 13:13), so His act creates an obligation for others to follow. If humble servanthood is not "beneath" the Son of God, it is not "beneath" any person who claims to follow Him (John 13:15–16).