What does Matthew 1:4 mean?

Matthew is tracing the line of Jesus from Abraham through David all the way to Joseph and Mary. During this time, his genealogy was very likely verifiable in Israel's historical records, in addition to several books in the Old Testament Scriptures. Ancestry was key to Jewish identity and even their worship. In other words, Matthew's Jewish readers would not likely have disputed any of the lineage he has presented so far.

In addition, it was common in a genealogy such as this to skip generations as a shorthand and for easy memorization. So it may be that when Matthew writes that Ram was the father of Amminadab (as does 1 Chronicles 2:10), he means that Ram was a direct ancestor of Amminadab and not necessarily his first birth father. This is likely the same Amminadab and Nahshon mentioned as being the father-in-law and brother-in-law of Moses' brother Aaron in Exodus 6:23. That means that four generations and about 400–450 years passed from Judah's son Perez to Amminadab.

Amminadab, though, was the direct father of Nahshon, called the "prince of the sons of Judah" in 1 Chronicles 2:10. Nahshon was the father of Salmon.