When they joined battle, Heb. when the battle was spread, i.e. when the two armies had drawn forth themselves into military order, and put themselves into the usual posture for fighting, and began to fight in their several places.
Related Commentaries of 1 Samuel 4:2
1 Samuel 4:2
2 And the Philistines put themselves in array against Israel: and when they joineda battle, Israel was smitten before the Philistines: and they slew of the army in the field about four thousand men.