What does Psalm 64:4 mean?

Not only were David's enemies telling lies (Psalm 64:1–3), but their slander was brought out by surprise. These opponents used well-timed criticisms the same way an assassin might ambush their target by firing arrows at them from a hidden spot. David experienced this in a physical way when king Saul suddenly threw a spear with the intent to kill (1 Samuel 18:10–11). The rebellion of David's son, Absalom, also came so quickly that it forced a hasty evacuation (2 Samuel 15:13–14). These enemies do so without concern that they are immoral and subject to judgment (Psalm 10:7–11).

It's possible that Saul's appointment of David to a fighting position was a sneaky attempt to have him killed (1 Samuel 18:12–13). But the Lord protected David and gave him great military success (1 Samuel 18:14). David's downfall was the goal behind Saul's dangerous, bizarre request regarding David's marriage to Michal (1 Samuel 18:25). In fact, the marriage, itself, was part of Saul's plot to weaken David (1 Samuel 18:21). First Samuel 19 makes it clear that Saul lied to his son Jonathan when he said he would not put David to death. Despite the king's words, when David returned to Saul's palace, Saul tried to murder him again (1 Samuel 19:10). Later, he sent spies and assassins to David's house to kill him (1 Samuel 19:11).