This scene is typical of how most cultures, not only the Hebrews, have understood the idea of witnessing. These ten elders of the city—and, if we read between the lines in verse 11, all the people who were drawn to this event that Boaz set up—observed the negotiations and the transaction of the sandal between Boaz and the unnamed near kinsman. If there were ever a need for proof that Boaz had indeed jumped through all the legal hoops through which he needed to jump to procure the land of Elimelech and the hand of Ruth, he had ten expert, irreproachable witnesses from among the elders of the people. In fact, he had probably dozens more who had seen all this take place because it most likely took place at the gate of the city.
So, many people were able to see what had transpired and could testify that everything had been done above-board. In a way, these people functioned like today's notaries who witness a legal transaction, put their seal on a document, and sign it, verifying that, "Yes, I indeed saw this transaction take place, legally and above-board, etc." This is how witnessing is done, and this is what many, if not most, of the occurrences of the Old Testament Hebrew words for "witness," 'ed and 'ud (the noun and the verb, respectively), connote.
What happened here in Ruth 4 is very interesting in the fact that Boaz is a type of Christ. Boaz here chooses ten elders—Jews—respected men of the town to witness what he did. Remember, since this took place in Bethlehem, these Jews were probably kin of David. In fact, Boaz himself was David's great-grandfather, but these people were all one big extended family, the family of Judah. Boaz took ten of them, ten men whose eyewitness testimony could not be gainsaid in any way, and these men then witnessed his redemption of the land and Ruth.
What is interesting is that Jesus did exactly the same thing, except that He chose twelve men of Judah from Galilee. They would do the same for Him, telling all who would hear that He had indeed redeemed His people. Luke 24:44-49 shows that this is exactly what He did. While the normal, legal idea of witnessing appears in the New Testament, Jesus makes use of it to confirm the facts of His life and death to the whole world through His witnesses, the apostles.