Verse 1 said that Pelishtim took Aron ha'Elokim. Why was this repeated?
Malbim: They took it from Ashdod, and brought it to Beis Dagon.
What was Dagon?
Rashi: It was an image in the form of a fish.
Radak (4): From its navel and below it had the form of a Dag (fish). This is why it is called Dagon. From its navel and above it had a human form. We find that it had hands (verse 4)!
Why did they put it by Dagon?
Malbim citing R. Yochanan, in Midrash Shmuel 11:4: This was to honor it. They said, they are both gods - it is proper that a god dwell by a god. 1
Malbim citing Reish Lakish, in Midrash Shmuel 11:4: They said, this conquered and this was defeated - the defeated should serve the victor.
Reish Lakish (11:4): This is wrong. If so, why were they punished?! (Perhaps it was for equating Aron ha'Elokim to folly! - PF)