1. PREPARATION
David was simply being prepared. What if the Philistines attacked him after he killed Goliath? How would he have defended himself? David was simply being prepared when he took the four additional stones. Also, he couldn't have known that one stone would be enough to kill the giant. God had not promised that David would kill Goliath with the first stone.
2. AS HE WAS AND WHATEVER TOOLS & SKILLS HE HAD.
David was a Shepherd by profession. David had experience in defending the sheep he shepherded and guarded with his sling and stones. David told Goliath that he (Goliath) came with spear and sword, but his weapon was his supreme faith in God Yahweh (1 Samuel 17:37) David had faith, that YAHWEH WILL MAKE A WAY. He trusted God with all his heart, believing that God would tell him exactly what to do and how. And so He did.
3. FIVE STONES FOR FIVE BROTHERS???
JUST A THEORY: Some speculate that David chose five smooth stones because Goliath had four brothers, and David was readying himself to dispatch all five giants. This theory is based on 2 Samuel 21:15–22. That passage lists four very large Philistines who were related to Goliath in some way: Ishbi-benob, Saph, Goliath, and an unnamed giant with six fingers on each hand and six toes on each foot. “These four were born to the giant in Gath, and they fell by the hand of David and by the hand of his servants” (2 Samuel 21:22).
David probably picked up five smooth stones to kill Goliath because Goliath had four relatives. In that ancient culture, there was a phenomenon called "blood revenge." If a person were to harm another person, often times a relative would come and take vengeance on the offender. Then, it might occur where retaliation was made, and then more relatives got involved on both sides, and an escalation of violence often followed. David knew that if he would defeat Goliath, he would also have to deal with his relatives.
"And there was war at Gath again, where there was a man of great stature who had six fingers on each hand and six toes on each foot, twenty-four in number; and he also had been born to the giant. 21 And when he defied Israel, Jonathan the son of Shimei, David’s brother, struck him down. 22 These four were born to the giant in Gath, and they fell by the hand of David and by the hand of his servants," (2 Samuel 21:20-22).
4. CREDIT, GLORY, PRAISE & POWER IS OF YAHWEH. NOT OF DAVID.
Whether it took one stone or five, David recognized that the power was not in his sling but in the Lord of hosts. As David wrote later in Psalm 21:13, “Be exalted, YAHWEH, in your strength; we will sing and praise your might.”
5. DAVID HAD WISDOM OF GOD.
David did not just pick any type of rock from the ground. He chose them carefully. He knew they would have to fly through the air towards the giant. If the stones were flat, they would resist the wind and might not strike the target at all. You will remember that these stones were taken from the brook or moving stream. They were smooth and aerodynamically sound because the water was constantly moving over them. God tells us that His Holy Spirit is like water (John 7:37-39).