Adam Clarke on Ruth 3:4

Uncover his feet, and lay thee down - It is said that women in the East, when going to the bed of their lawful husbands, through modesty, and in token of subjection, go to the bed's foot, and gently raising the clothes, creep under them up to their place. See Calmet.

On the whole, we must say, had not Boaz been a person of extraordinary piety, prudence, and continence, this experiment might have been fatal to Ruth. We cannot easily account for this transaction, probably Naomi knew more than she revealed to her daughter-in-law. The experiment however was dangerous, and should in no sense be imitated.

He will tell thee what thou shalt do - The Targum reads the clause thus: Thou shalt ask counsel from him, "and he shall tell thee what thou shouldest do."