There be three things which are too wonderful for me, yea, four which I know not:

Wesley's Notes

30:18 Wonderful - The way whereof I cannot trace.

Jamieson-Fausset-Brown Bible Commentary

18-20. Hypocrisy is illustrated by four examples of the concealment of all methods or traces of action, and a pertinent example of double dealing in actual vice is added, that is, the adulterous woman.

Matthew Henry's Concise Commentary

30:10 Slander not a servant to his master, accuse him not in small matters, to make mischief. 11-14. In every age there are monsters of ingratitude who ill-treat their parents. Many persuade themselves they are holy persons, whose hearts are full of sin, and who practise secret wickedness. There are others whose lofty pride is manifest. There have also been cruel monsters in every age. 15-17. Cruelty and covetousness are two daughters of the horseleech, that still cry, Give, give, and they are continually uneasy to themselves. Four things never are satisfied, to which these devourers are compared. Those are never rich that are always coveting. And many who have come to a bad end, have owned that their wicked courses began by despising their parents' authority. 18-20. Four things cannot be fully known. The kingdom of nature is full of marvels. The fourth is a mystery of iniquity; the cursed arts by which a vile seducer gains the affections of a female; and the arts which a vile woman uses to conceal her wickedness. 21-23 Four sorts of persons are very troublesome. Men of low origin and base spirit, who, getting authority, become tyrants. Foolish and violent men indulging in excesses. A woman of a contentious spirit and vicious habits. A servant who has obtained undue influence. Let those whom Providence has advanced from low beginnings, carefully watch against that sin which most easily besets them.