What does Psalm 66:4 mean?

The psalmist indicates that the whole world worships God and sings praises to Him. This is phrased in the present tense. Of course, when the psalm was written, only Israel consistently worshipped the One True God. To an extent, all worship reflects the sense that God exists and is worthy of praise (John 4:22). More likely is that this verse is looking to the future.

Though nations oppose God's rule now, this situation will be resolved. During the chaotic period following Jesus' removal of the church from the world, a remnant of Jews will believe in Christ. That group will herald the gospel to the ends of the earth (Matthew 24:14; Revelation 7). At His return to earth Jesus will judge the nations. The nations that rejected the saved Jews' message will be consigned to eternal punishment, but those who accepted their message and believed on Jesus will be accepted into His kingdom (Matthew 25:31–46).

Despite the broad language, these judgments are made at the individual level. Heaven includes ransomed people of God from every tribe, language, people, and nation (Revelation 5:9–10; 7:9; 21:26).