John Gill Commentary 1 Samuel 4:7

"And the Philistines were afraid, for they said, God is come into the camp. And they said, Woe unto us! for there hath not been such a thing heretofore." — 1 Samuel 4:7 (ASV)

And the Philistines were afraid
When the spies returned, and reported to them the reason of the shouting:

for they said, God is come into the camp ;
into the camp of Israel, because the ark represented him, and was the symbol of his presence; and these Heathens might take the ark itself for an idol of the Israelites; the Targum is, "the ark of God is come"

and they said, woe unto us ;
it is all over with us, destruction and ruin will be our case, victory will go on their side now their God is among them:

for there has not been such a thing heretofore ;
if by this they meant that the ark had never been in the camp of Israel before, they were mistaken; and it is no great wonder they should, being not so well acquainted with the affairs of Israel, and their customs; or rather, it was not so yesterday, or three days ago, when they were defeated; there was no shouting then: or the state of the war is altered; before we fought with the men of Israel, but now we must fight with the God of Israel also.