A journal article Telescoping N + 1 Patterns in the Book of Amos by Robert H. O'Connell published in Vetus Testamentum Vol 46 (Jan 1996) proposes how the 8 judgment speeches are arranged "according to a stepwise pattern of escalation that involves the telescoping of N + 1 groupings (where "N" represents a number, usually 3 or 7)."
General observations that support this thesis are:
- the speech forms in Amos appear to have been arranged into N + 1 groupings
- the final (or +1) speech form in each such grouping usually presents some rhetorical surprise that departs from the pattern established by the other speech forms in its group
- until the end of the book, the concluding speech form of each N + 1 grouping serves as a transition to all the succeeding N + 1 groupings, resulting in a telescoping pattern of development throughout the book.
As for the "three transgressions of [PN], and for four" pattern, the article says:
Similar N ‖ N+1 patterns are used commonly enough in Hebrew, Ugaritic and Akkadian poetry to warrant little need to explain or justify this attestation of the device as another instance of ascending numerical parallelism.⁴ What is significant for my purposes is to note that in Amos i 3 - ii 16 there are a total of eight (8) judgment speeches that contain this formula.⁵
⁴ For various treatments of the ascending numerical pair as a device in Semitic poetry, see the appendix at the end of the present article
An article from the GotQuestions.Org website Why does Amos keep repeating “for three sins . . . even for four” in chapters 1–2? explains (emphasis mine):
The phrase “for three sins . . . even for four” is a common phrase in Amos (1:3, 6, 9, 11, 13; 2:1, 4, 6). Used a total of eight times in the book, these words play a special role in the way Amos communicates sin and judgment. “Three sins” represents fullness or completeness; “four” represents an overflow or a sin that is the tipping point for God’s judgment. The word sins or transgressions in Hebrew specifically refers to “rebellions.” The first two chapters of Amos contain eight messages against the nations, including Judah and Israel, condemning them for their rebellion against the Lord.
Similarly, in the Daniel chapter of The Literary Guide to the Bible (edited by a professor of Hebrew and comparative literature Robert Alter) the author Shemaryahu Talmon notes (emphasis mine):
[page 347] Daniel shares with other biblical writings a predilection for the ascending numberical pattern 3 + 1, observable in other ancient Near Eastern literature. Whatever the roots of this pattern, it signifies a basic "complete" unit of three, topped by a fourth of special standing and importance.
[page 348, last paragraph] The pattern 3 + 1 finds a most salient expression in Amos's oracles against foreign nations (Amos 1:3-2:3) and against Judah and Israel (Amos 2:4-16). The phrase "for three transgressions . . . and for four," which recurs in every instance, shows the fourth to be more damnable than the preceding ones: "Thus saith the Lord ... I will not turn away the punishment thereof" (Amos 1:3, 6, 9, 11, 13; 2:1, 4, 6). In this as in many other instances, the quintessence of the pattern is to be sought in the "fourth" item in which the series culminates, and which is intrinsically different from the preceding unit of "three" which serves as its antithesis. Therefore, the component "three" cannot be interpreted as referring to a precise number, but rather should be viewed as a schematic literary figure. ...