First, the first four stanzas of Milton's poem have a distinct rhyme scheme. The rhyme scheme is an adaptation of the rhyme scheme in Spenser's The Faerie Queen. In Spenser's poem the stanza rhyme scheme is ababbcbcc. In Milton's poem the rhyme scheme is ababbcc. The two poems have similar rhyme schemes except Spenser's poem has an extra cb.
Second, the most common feature of the first four stanzas is the striking allusion. In the stanza, Milton speaks of the "son of Heaven's eternal King" born of a "virgin mother" bringing "redemption from above." The "King of Heaven" is the Christian God. This is a quote from Daniel 4. The virgin mother is Mary, mother of Jesus. This is a prophecy of the birth of Christ from Isaiah 7 fulfilled in Luke 2. The "redemption from above" is the redemption written of in Romans 3:27. The "holy sages once did sing (they prophesied)" about Christ's releasing the elect of death from the Fall (Genesis 3). These prophecies are in Isaiah 9 and 40, among other places. They were fulfilled in Christ's death and resurrection, and God now works a "perpetual peace" in His elect.
The second stanza is much like the first, with allusion as the main feature. The "glorious form" and "light insufferable" are symbolic of God. Exodus 33:20 says no man shall see the face of God and live. Here Milton specifically writes about the Son, Jesus Christ. Milton says He sat in "Trinal Unity" at "Heaven's high council-table." "Trinal Unity" refers to the Christian doctrine of the Trinity. All three are separate and distinct Persons of one God. They are in perfect unity. Finally,