Paul went on to say in verse 9 that Christ did not only ascend up into heaven, but that He also first descended into the lowest parts of the earth, so that the Scripture could be fulfilled that He is above all things and fulfilled all things [Eph 1:23].
Paul was simply explaining the unity of our faith, the prophecy of the Word, and the gifts of the ministry were all to the end that we would all be one in Jesus [see John 17].
Paul is referring to the sequence of events that happened between "it is finished" on the cross and Jesus seated on the throne. Many Christians are not informed in what the Scripture says about these events, but they are vitally important to our faith. I will try to reproduce here what I taught in several meetings and as part of a WVCU class.
What happened from the cross to the throne?
1. Jesus died spiritually when He became sin [Matt 27:46; Psalm 22; Psalm 51:9; II Cor 5:21; Eph 2:1-9]. To "die" doesn't mean to "cease", but to be separated from God.
2. Until Jesus died spiritually, it was not possible for Him to die physically. Jesus died spiritually so He could enter hell the way that men do: as the result of sin. If He entered as deity He couldn't really be our substitute. Therefore, Jesus took on sin, died a physical death, and descended into hell [Eph 4:9-10; Acts 2:23-27; Psalm 16:8-11; Acts 13:29-34; Psalm 88].
4. After being justified, Jesus defeated satan and took from him the keys of death and hell [Col 2:15; Heb 2:14; Rev 1:18; Mark 16:17-18; Matt 28:18-20].
5. Jesus preached to all those who were in hell, both the lost and those justified by faith. In the OT, the place was called "Sheol" or "Abraham's bosom". The Greeks referred to the place as "hell". The location of this place was in the center of the earth. In Luke 16, the story of the rich man and Lazarus explains the situation. Abraham was waiting for the Messiah. When Peter quoted David [Acts 2, see above], David was referring to waiting in this place for the Messiah.
6. The portion of hell that was reserved for the justified was also referred to as "Paradise". Jesus took Paradise with Him into heaven when He ascended. The fact that the Word says that "captivity" -- those who had passed on and were waiting in Abraham's bosom -- were led "captive" means that that they aren't up in heaven celebrating while we're still down here. They are entered into the "rest of the Lord" [see Heb 4 and Psalm 127:2]. So when Jesus ascended, He relocated Paradise into heaven. The place of torment remained in the lowest part of the earth [Eph 4:8; II Cor 12:2-4; John 3:13-15; II Cor 5:6-8; I Thess 4:13-15].
So this is what happened from the cross to the throne. In Ephesians 4, Paul was referring to this sequence of events, filling in what happened in the spiritual realm to give unity to our faith as well as to make the case for unity in our faith.