This chapter contains the song of Deborah.
From what I heard when I was researching the dating of Judges, this song is supposed to be one of the oldest parts of the book based on linguistic analysis. I don't think you can really tell just by reading the English translation though, because content-wise it fits in pretty closely with what we just read.
The song describes, in flowing terms, how Israel was oppressed in the days of Jael (v. 6-7) until the mother Deborah arose, an army was gathered, and a battle fought against Sisera. After Sisera's defeat, his mother stands in opposition to Deborah, the mother of Israel, and Jael, the most blessed woman who slays Sisera. Sisera's mother, for her part, worries at the delay of her son's chariot bringing back news of the victory. She wonders if Sisera is late because he is gathering spoils, but instead we had already learned that Barak was gathering the spoils to himself from his victory (v. 12).
So I'd say that's the first theme of Deborah's song: drawing a contrast between the valorous and victorious mothers of Israel against the distraught mother and princesses of Sisera. It is common in biblical songs for women to rejoice or mourn at the results of various battles. What is uncommon is Deborah's participation as a leader of Israel and Jael's prominent role in slaying Sisera. The weapons that Jael used, too, are the weapons of a homemaker, not a warrior.
The second theme I'd like to point out is the contrast between the participating tribes and those who did not fight against the Canaanites (verses 14-18). This chapter lists six tribes as participating: Ephraim, Benjamin, Makir (a clan of Manasseh), Zebulun, Naphtali and Issachar. I'm not sure exactly what it's trying to say about Reuben, but the consensus seems to be that Reuben did not participate in the battle. Gilead (a region of Manasseh east of the Jordan) did not fight, nor did Dan and Asher.
This is a more expansive list of participating tribes than what we read in the last chapter (where Deborah commanded Barak to gather men from Zebulun and Naphtali), but I don't think this is a significant discrepancy. I would guess that Zebulun and Naphtali are the most significant tribes present for the battle (since they are the two tribes honored in v. 18 of the song), but it's not contradictory to suggest that the other tribes named here (Ephraim, Benjamin, etc.) sent smaller contingents that simply didn't merit inclusion in Judges 4.
The third major theme is in verses 19-23, which recounts the LORD's divine assistance on behalf of Israel. This section begins by telling us that "the kings" are fighting in Taanach, but it quickly elevates to the heavenly realm. This shows us that while kings were battling on the earth, there was a corresponding battle going on in the heavens as well.
The stars and the river Kishon fought against Sisera. The stars are a common metaphor for angels, in this case fighting "from heaven" to influence the battle on earth in Israel's favor. This creates a dual suggestion of both supernatural power fighting for Israel, but also the natural world (in the literal stars and the literal river Kishon) fighting on behalf of Israel. Many powers, then, are aligned to ensure Israel's victory against a stronger army (symbolized for its part by the iron chariots and the dashing hoof beats of the mighty horses, v. 22). The root source of Israel's assistance, however, is the angel of the LORD, who is bringing his power to bear against their enemies. It's interesting that the thundering chariot, a symbol of Sisera's power, is exactly what is delayed in v. 28 to the dismay of his mother.
The last thing I would like to point out is the use of poetic repetition, something that I have discussed before, beginning all the way back in Gen 1:27 (which repeats the words "God", "created" and "image"). In this chapter we see repetitions all over the place, but some examples are v. 7 ("until I arose"), v. 11 ("the righteous deeds"), v. 12 ("awake, awake!"), and my personal favorite, v. 27: "He bowed, he fell; where he bowed, there he fell." There are a bunch more, in nearly every verse of this chapter, so I won't list the rest.
There are a few other minor details that I don't really understand. what is Meroz from v. 23? Is it a city in Israel? Also, I don't really understand why there are references to the LORD marching out of Seir/Edom in v. 4. So I will gloss over these details. :) I think I have succeeded in mostly capturing the intended sense of the song with the three themes I listed above.