Should we expect Ephesians to have a well defined structure?
Ephesians was almost certainly written by Paul dictating his letter to a scribe. This was normal practice, and we know he certainly used this method for Galatians, to which he appended a postscript in his own writing (Gal 6:11).
The implications are significant. Those who dictate letters for secretaries know just how difficult this is, especially when the ideas are complex: usually the draft needs lots of alterations! But Paul could not make amendments: both the materials and labour of having a letter written by a scribe were expensive.
Psalm 12:6 says the words of the LORD are like silver refined in a furnace seven times. So we can be sure that the words of these letters were carefully drawn together in Paul's mind before they were committed to writing. To do this the underlying ideas must be carefully structured, which makes it memorisation relatively easy. Jeremiah was able to reproduce his scroll again (Jer 36:28) after his first scroll (Jer 36:2-4) had been destroyed by the king (Jer 36:20-23). Doubtless Paul could have done the same with Ephesians.
The parallels
Ephesians divides into two halves, chapters 1-3 deal with principles, while 4-6 show how those principles are worked out. The ordering of the topics in 4-6 roughly parallels the order of topics in 1-3. The following breakdown is only indicative, to provide a framework for personal study of the verbal and other parallels: naturally there is considerable transition and overlap between each topic and the next, and at all stages Paul draws on all the preceding argument.
Eph 1:3-14 (& 2:14-18) // Eph 4:1-10 Eph 1:3-14 reveals God's intention to unify everything in Christ (Eph 1:10), and to bless the church through Christ. This is also the focus of Eph 4:1-10, which begins to show how Christ filling all things (Eph 4:10) is achieved - by Christ giving gifts to men (Eph 4:8). The promised Holy Spirit (Eph 1:13-14) parallels the gift of Christ (Eph 4:7)
Eph 1:15-23 (& 2:19-22) // Eph 4:11-16 Paul prays that God's people
will have their eyes opened to see this. Eph 4:11-14 reveals that the means
appointed to bring about the necessary understanding, and out of it the unity
of faith, is the gift of Christ to His church of the four anointed ministries
(apostles, prophets, evangelists, pastor/teachers) (also Eph 2:20-22).
In Eph 1:15-23 the church also revealed as body of Christ (Eph 1:23): Eph
4:15-16 (and onwards) shows how this body is to function.
Eph 2:1-10 // Eph 4:17-5:14 Eph 2:1-10 deals with the contrast in
position between believer & unbeliever (the "sons of disobedience" Eph
2:1, 5:6), and Eph 4:17-5:14 with the consequently required change in moral
outlook and practice.
Eph 2:11-22 // Eph 5:15-6:9 The change in relationships exemplified
in the uniting of Jew & Gentile "in Christ" in Eph 2 to have the "same"
everything (4:4-6, cf 2:14,16) has parallels in other difficult relationships
(5:21-6:9). The principles are derived from the divine order for relationships
(see also 1Cor 11:1-9): this divine origin and order is seen most clearly
in Eph 5:22-33.
The right temple (2:21-22) is expressed in right worship (5:19-21).
Eph 3 // Eph 5:21-6:20 The mystery of unity of all things in Christ, previously hidden (Eph 3:5-6), is to be revealed through the church to the principalities and powers (Eph 3:10). At the heart of the divine mystery is the love/ submission relationship between Christ and His church (Eph 3:16-21; 5:32). In addition to its outworking in divinely ordered relationships, its powerful reality is demonstrated in the ability of the church to cope with direct attacks by the Enemy (Eph 6:10-19, cf Eph 3:13), and declare the mystery (Eph 6:19-20).
Rightly understanding the underlying structure is essential for correct interpretation: many "bottom-up" interpretations are eliminated once the thrust of the argument is appreciated.