John Gill Commentary 1 Samuel 6:4

"Then said they, What shall be the trespass-offering which we shall return to him? And they said, Five golden tumors, and five golden mice, [according to] the number of the lords of the Philistines; for one plague was on you all, and on your lords." — 1 Samuel 6:4 (ASV)

Then said they, what shall be the trespass offering which we shall return to him ? &c.] They paid a great deference to their priests and diviners, and were willing to be directed in all things by them; being ignorant of what was most proper in this case, and might be acceptable to the God of Israel:

they answered, five golden emerods, and five golden mice ; &c.] images of these made of gold, as appears from the next verse; the reason of the former is easy, from the above account of the disease they were afflicted with; but of the latter no hint is given before: indeed in the Vulgate Latin and Septuagint versions of (1 Samuel 5:6) is inserted a clause, that ``mice sprung up in the midst of their country;'' which is not in the Hebrew text, nor in the Chaldee paraphrase; yet appears to be a fact from the following verse, that at the same time their bodies were smitten with emerods, their fields were overrun with mice, which destroyed the increase of them; wherefore five golden mice were also ordered as a part of the trespass offering, and five of each were pitched upon:

according to the number of the lords of the Philistines ; &c.] who were five, and so the principalities under them; see (Joshua 13:3)

for one plague was on you all, and on your lords ; &c.] the lords and common people were equally smitten with the emerods, and the several principalities were alike distressed and destroyed with the mice; and therefore the trespass offering, which was a vicarious one for them, was to be according to the number of their princes and their principalities; five emerods for the five princes and their people smitten with emerods, and five mice on account of the five cities and fields adjacent being marred by mice.