Commentaries:
"Spirit," referring to the essential force within God, is used to help us understand the attitude motivating His actions. What we see on the outside, externally, is judgment; but spirit is an internal, invisible force, impelling, motivating, generating His judgment, which is "burning," representing a punishing, purging, purifying destruction. So in God, His spirit motivates Him to judge. This is how "spirit" is used in God's Word.
John W. Ritenbaugh
The Holy Spirit and the Trinity (Part Two)
The Branch mentioned here is a frequent symbol of Jesus Christ, who is of the God Family but also of the fruit of the earth, meaning He is both Godkind and humankind. Notice that only those chosen and rescued by God benefit from the enhanced production of fruit.
Isaiah 11:1 explains the Branch more fully: “And there shall come forth a Rod out of the stem of Jesse, and a Branch shall grow out of his roots.” The Branch descends from the line of Jesse, the father of David. Jeremiah 23:5 and 33:15 both refer to “a Branch of righteousness” of Davidic lineage, and in Revelation 22:16 Jesus himself affirms, “I am the Root and the Offspring of David.”
Bill Onisick
The Branch of God's Planting
He is called "the Branch of the Lord"—He is God's descendant! He is the Son of God, not just the son of the man David! Yet this verse also describes Him as "the fruit of the earth," meaning that, though He is God, He is also from mankind. He could claim full descent from both Godkind and humankind!
Richard T. Ritenbaugh
The Branch
Other Forerunner Commentary entries containing Isaiah 4:2:
Zechariah 3:1-5
Luke 1:78