Here's one way of dividing the text that I shall follow:
Synopsis
Initial greeting (1:1,2)
-
Word of Endorsement (1:3-14)
-
Word of Exposition (1:15-23)
-
Word of Explanation (1:24-2:5)
-
Word of Exhortation (2:6-23)
-
Word of Expectation (3:1-4:6)
Final greetings (4:7-18)