Paul instructs the Colossian church concerning the nearby church in Laodicea. He then picks up the pen and adds his own personal greetings.
Laodicea (15-17): Paul gives them three instructions regarding the believers in Laodicea. They are to greet them and ensure that one particular part of the church isn't excluded: Nympha and the church at her house. Also, the saints from these cities are to exchange and read each other's letter from Paul. Their letters were relevant to both. Finally, they must exhort Archippus, who is in Laodicea, to fulfill his ministry. Paul does not speak to him directly. Instead, he sees the Colossian believers positioned to effectively urge him to his weighty responsibility in the Lord. One must fully carry out their God-given duty!
Greetings (18): Paul concludes with his trademark hand-written greeting. He also adds, "Remember my imprisonment." They should keep calling to mind the great price with which the preaching of Christ comes to them. The gospel is precious! He then wishes them God's grace as he did at the beginning of his letter. How deeply we all need God's continued unmerited favor!
 ESV accurately starts 4:16 and 4:17 with the conjunction "and [kai (καί)]." These sentences form a single section. The Colossians were to visit Laodicea, then greet the saints, do a letter-swap and exhort Archippus.
 Churches regularly met in homes (Acts 2:46; Rom. 16:3-5, 23). Multiple such house gatherings made up the one Laodicean "church" (4:16). Paul ensures that one particular host and her house are not overlooked.
 The Laodicean letter is either "Philemon" or "Ephesians." The former is likely, since Philemon is in Laodicea alongside Archippus who is there (Phm. 1-2). The reading was a public reading (cf. 1 Thes. 5:27; 1 Tim. 4:13).
 Paul did not include this instruction in his letter to Philemon, which included Archippus as an addressee. This was a unique circumstance where Archippus needed to hear this exhortation from the Colossian saints.
 Paul included a signature like this in his other letters (1 Cor. 16:21; Gal. 6:11; 2 Thes. 3:17).
 "Remember" is imperfective. They are to continually remember his imprisonment for their sake.
 Paul's suffering was for them, so that they would have God's Word (1:24-25), the truth about Christ (2:1-2).
 Cf. 1:2. Paul often ends his letters this way (1 Cor. 16:23; 2 Cor. 13:14; Gal. 6:18). Believers need God's grace.