One could paraphrase verse 3, "When God gave me (David) this promise that my descendant (Jesus Christ) would rule forever, trusting in that promise gives me the strength and boldness to trust in You and do Your will for my life." For us, trusting in God's promises gives us the ability and the strength to face whatever it is we have to face. Someone said it well, 'We don't win in life by trying harder. We win by relying upon His strength to deal with whatever it is we have to deal with.'
I've always been a big believer that we don't have to strain to hear God speak to us. He is more than capable of speaking to us however, whenever and wherever He wants to communicate. I'm also convinced the reason God doesn't speak to us more often, is simply because it is our move, that is, He is waiting for us to obey what He has already told us before He can say what it is He wants us to do next.
So how do we know when God has spoken? God speaks to us as we read and study our Bible. We must continually pray for God's will and direction in our life.
God did speak to David and he figured out that it was God speaking to him. David trusted that promise and it gave him the strength to be obedient to God. The point for us is that we can trust in that same promise made about David's descendant (Jesus Christ) who one would day rule forever. As we walk in obedience to God's word, He will make His will known to us just as He did to David.
David is acknowledging in verse 4 that because this descendant (Jesus Christ) of his will rule forever, then it is David's wish that all the great leaders of the world acknowledge that fact. It is David's hope and prayer that all leaders and all people acknowledge a single God that rules over this world and also one day that this God will rule the world from this future temple in Jerusalem.
A practical implication for the Christian today is simply that it should be our desire for the leaders of this world to acknowledge there is a single God who ultimately rules over this world. In effect, this is a prayer David made and should be a prayer of ours as well. This is really a prayer for peace by praying that people's hearts turn to God.
When we pray for others to turn to God, it is about the realization that others become aware there is a God who is in charge of all things. Praying for our leaders (called "kings" here) is in effect praying for our leaders to become aware that God wants to guide over every aspect of their lives as well as our lives.
What David wants is for the great men of his world not to honor David, but to acknowledge God as ruling over their lives. Just as the theme of these verses is about knowing God, so David wants us to spread that prayer request to others.