Adam Clarke on 1 Samuel 4:2

Put themselves in array - There is no doubt that both the Philistines and Israelites had what might be called the art of war, according to which they marshalled their troops in the field, constructed their camps, and conducted their retreats, sieges, etc.; but we know not the principles on which they acted.

They slew of the army in the field about four thousand men - This must have been a severe conflict, as four thousand were left dead on the field of battle. The contest also must have lasted some considerable time, as these were all slain hand to hand; swords and spears being in all probability the only weapons then used.