‘ And all the people who were in the gate, and the elders, said, “We are witnesses.”
Then all the people in the gateway gathered together with the elders, and declared ‘we are witnesses'. This parallels Boaz's statement in Ruth 4:9, ‘you are witnesses this day', and combined with the twofold ‘you are witnesses' in the Ruth 4:9-10 it indicates a threefold, and therefore complete, testimony. From then on the legality of what Boaz was doing could not be questioned, for there would be many witnesses who could declare that he had acted rightly. The whole episode underlines his integrity, and his determination to do what was right, indicating why he was a chosen vessel of YHWH.
“YHWH make the woman who is come into your house like Rachel and like Leah, which two did build the house of Israel, and do you worthily (or ‘powerfully') in Ephrathah, and be renowned in Beth-lehem, and let your house be like the house of Perez, whom Tamar bore to Judah, of the seed which YHWH will give you of this young woman.”
We are not told who spoke these words, but they were presumably spoken by one of the leading elders on behalf of all the people. It is a plea to YHWH on behalf of Boaz and his house. The initial plea is that Ruth, having come into Boaz's house, will be as fruitful as Leah and Rachel who, together with their maidservants, came into Jacob's house, and produced, the twelve sons of Jacob/Israel, thus laying the foundations for the multitude of the house of Israel. These women whom Jacob married were, although related to him, from outside the tribe, and came from a family which worshipped false gods. And yet God used them as builders of the house of Israel, just as He would now use Ruth. It is also a plea that Boaz may have many influential sons. And the attention then turns directly on Boaz with the desire that he will be prominent in righteousness (‘do -- worthily') or power (‘do valiantly' - Psalms 60:12), in Bethlehem Ephratha (see note on Ruth 1:2), and that his house will be like the house of Perez, the son whom Judah begat through Tamar, and this as a consequence of the ‘seed which YHHW will give him' through Ruth. This is preparatory to the final listing of the descendants of Perez in Ruth 4:18-22 (which the chiasmus makes clear is an essential part of the book). The word for ‘worthily' is chayil, which is used of Boaz in Ruth 2:1 and of Ruth in Ruth 3:11. Thus the virtues of both are to be reflected in their future lives. The writer may have intended us to see this as being finally fulfilled to the fullest extent in the life of David. So the basic plea is that Boaz will be both successful and fruitful, and will have sons who are equally prominent.
The plea that his house might be like the house of Perez almost certainly had in mind the fact that Perez could be seen as parallel to the son who would be born to Boaz and Ruth in that he was the eldest son born as a consequence of Judah impregnating Tamar when he was (unknowingly) acting as her near kinsman in begetting a son for her dead husband (Genesis 38). Furthermore while Tamar is not said to be a Canaanite, Judah certainly mistook her for a Canaanite harlot, and she was married to one of Judah's sons who was born as a result of his marriage to a Canaanite. Perez was thus seen as the ‘son' of a half-Canaanite. Yet this had not prevented his house being fruitful. Indeed, as we soon learn, it was so fruitful that it produced King David. (But this last fact would not, of course, be known to the elders. It was his birth through a near kinsman, and his foreign connections, that in their eyes paralleled what would happen to Ruth).