What does Acts 4:20 mean?

Shortly before Jesus ascended into heaven, He told His disciples, "You will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth" (Acts 1:8). This isn't a command, it's just a statement of fact. Peter will validate the salvation of the Samaritans (Acts 8:14–17) and Gentiles (Acts 10). John will write his Gospel (John 21:24). Paul will insist he has no choice but to preach the gospel (1 Corinthians 9:16).

Jesus told His disciples that as they preached, the authorities would persecute them. But He also explained how much they are worth to God, and to remember that their earthly tormentors may kill their bodies, but God forever holds their souls. He said that He would remain faithful to those who are faithful to Him, up to and including claiming them as His own before Father-God (Matthew 10:16–33).

By the power of the Holy Spirit (Acts 2:1–4), Peter and John have reached the place where Jesus wants them to be. They value their eternal souls more than their lives. They value Jesus' instruction more than their religious leaders. They trust God's plan more than they fear the ungodly authorities. They must speak, and they must speak what they know about Jesus. To obey the Sanhedrin's orders would be to directly disobey Jesus, and they are past the point where that is an option.