4:6 Paul exhorts the Philippians to pray about their circumstances instead of worrying over them. be anxious for nothing: although the same word in 2:20 describes Timothy's concern for the Philippians, here Paul uses the word to refer to worry. He prohibits the Philippians from worrying about their own problems. Instead they are to commit their problems to God in prayer, trusting that He will provide deliverance.
4:7 will guard: Paul's choice of a military term implies that the mind is in the battle zone and needs to be "protected by a military guard." Since the purpose of such a guard in a wartime situation is either to prevent a hostile invasion or to keep the inhabitants of a besieged city from escaping, the peace of God operates in the same way: to protect the mind from external corrupting influences and to keep the mind focused on God's truth.
[THE NKJV STUDY BIBLE Second Edition].
6 Believers should be prayerful instead of "anxious". This verb can mean "to be concerned about" in a proper Christian sense (cf. 2:20), but here the meaning is clearly that of anxiety, fretfulness, or undue concern (cf. Matt. 6:25-34). Paul is not calling for apathy or inaction, for as we make plans in light of our circumstances, it is our Christian privilege to do so in full trust that our Father hears our prayers for what we need. The answer to anxiety is "prayer", a word that denotes the petitioner's attitude of mind as worshipful. "Petition" denotes prayers as expressions of need. "Thanksgiving" should accompany all Christian praying, as the supplicants acknowledge that whatever God sends is for their good (Rom. 8:28). It may also include remembrance of previous blessings.
7 Paul attaches to this classic exhortation to pray the beautiful promise that when we turn from anxiety to prayer and thanksgiving, God will give us his own "peace." This peace is for those who are already at peace with God through justification by faith in Christ (Rom. 5:1). This peace of God "transcends all understanding," for it is not only sufficient for us but far surpasses human comprehension. It acts as a sentry to guard our hearts and thoughts from all anxiety and despair.
[NIV BIBLE COMMENTARY Volume 2: New Testament].
JESUS IS LORD.