What does Ruth 4:20 mean?

This genealogy of David is likely condensed. The time between Perez's father Judah to Nashon was over 400 years—from the time of Jacob's labor for Laban until the exodus (Exodus 12:40). Nahshon knew Moses (Exodus 6:23), whom historians think was born around 1526 BC. Historians think David was born shortly after 1040 BC. That's a lot of time for so few generations.

We don't know much about Amminadab except that Aaron married his daughter (Exodus 6:23); he would have been born in Egypt and lived most if not all of his life there. Nahshon was the chief of Judah during the exodus which made him the third most powerful Israelite after Moses and Aaron (Numbers 1:7; 2:3; 7:12).

Matthew 1:5 says that Salmon—"Sala" in Luke 3:32—married Rahab. Because Matthew lists Tamar and Ruth, two other foreign women in Jesus' line, this is likely Rahab the prostitute who helped the Israelites take Jericho (Joshua 2; 6).

Some scholars think there are missing generations between Salmon and Boaz, but unlike the list in Ruth 4:19, there are no other accounts with additional names. The line is possible if all the men from Salmon to Jesse were about 100 years old when they had their sons. It is certainly plausible, and even likely, there are missing men throughout the list. Israelites tended to reserve the fifth and seventh of a list for honor. The fifth here is Nahshon—a leader of his people. The seventh is Boaz (Ruth 4:21).

That there are ten names total is even more significant and gives further evidence the record is selective. "A list of ten generations is used to indicate a transition from one major era to another," According to Zvi Ron.Genesis gives ten generations from Adam to Noah (Genesis 5), then another ten between Noah and Abraham. In this case, the transition is from the time the Israelites went to Egypt until the time of the Davidic monarchy. Scholars say Samuel wrote Ruth; if so, he is documenting the legitimacy of David's monarchy over that of Saul.