What does Judges 21:4 mean?

Eleven tribes of Israel came together and nearly wiped out the twelfth tribe, Benjamin. They have done so in a zeal to purge evil from the Promised Land (Judges 20:12–13). God commanded they attack, but did not indicate they should exterminate every man, woman, and child. Israel's rage has left only six hundred men, barricaded and trapped, as the last of the tribe of Benjamin.

Making the situation even worse, the eleven tribes had made a foolish promise prior to the start of the war (Judges 21:1). This was a vow not to intermarry with the men of Benjamin. God gave such commands to Israel with respect to pagan Canaanites (Deuteronomy 7:1–5), but not among their own people. Yet vows made to God were taken very seriously (Deuteronomy 23:21–23).

This verse notes a second day of mourning and making sacrifices to God, seeking His will. On the first day, the people asked God "why" such a thing had happened, though they were the ones who chose to do it. Perhaps they hope that worshiping God will cause Him to break His silence and tell them how to save the tribe of Benjamin. God will not speak, leaving Israel to clean up their own mess, in this case.

The eleven tribes think of a partial solution, further complicated by yet another poorly-thought-out promise (Judges 21:5).