What does Psalm 39:4 mean?

Experiencing distress, David has resolved not to speak in the presence of certain evil people (Psalm 39:1–3). This might have been to avoid speaking inappropriately, possibly about God. Holding one's tongue is not easy (James 3:1–5), and David found his frustration growing. Rather than continue to hold in his angst, he speaks. But what he says is a prayer, to God, asking for a reminder of his own limitations.

David's question here is not about knowing the exact time or nature of his death. Instead, he is asking God to help him with humility. Wicked people frustrate David, and he does not want to respond sinfully. So, he asks God to work on his own heart. Being reminded of the short, fragile, imperfect nature of all humanity would make David less concerned over his current issues.

Proverbs 27:1 stresses the uncertainty of life by saying, "Do not boast about tomorrow, for you do not know what a day may bring." No one can predict when his life will end. It is best, therefore, to measure one's days (Psalm 90:12) and endeavor to do God's will every day (James 4:13–17). While life continues, the wise believer seeks God's kingdom and doesn't put unnecessary worry in tomorrow (Matthew 6:33–34). Jesus told a story about a rich farmer whose land produced an abundant harvest. Instead of consulting God to determine what he should do, the rich farmer assumed his future was secure. He told his soul, "Soul, you have ample goods laid up for many years; relax, eat, drink, be merry" (Luke 12:19). But that night the man died and was judged (Luke 12:20).