The New Testament uses several Old Testament verses which in their original context are about God and applies them to Christ. The use of Psalm 68:18 in Ephesians 4:8 is one of the better known ones. Let’s compare first the two passages:
17 The chariots of God are twice ten thousand, thousands upon thousands; the Lord is among them; Sinai is now in the sanctuary. 18 You ascended on high, leading a host of captives in your train and receiving gifts among men, even among the rebellious, that the Lord God may dwell there. (Psalm 68:17-18 English Standard Version)
4 There is one body and one Spirit—just as you were called to the one hope that belongs to your call— 5 one Lord, one faith, one baptism, 6 one God and Father of all, who is over all and through all and in all. 7 But grace was given to each one of us according to the measure of Christ’s gift. 8 Therefore it says, “When he ascended on high he led a host of captives, and he gave gifts to men.” 9 (In saying, “He ascended,” what does it mean but that he had also descended into the lower regions, the earth? 10 He who descended is the one who also ascended far above all the heavens, that he might fill all things. (Ephesians 4:4-10 ESV)
Note that this passage in Ephesians also has a common question based on Ephesians 4:9 which is related to the medieval idea of Jesus ‘harrowing hell’ during his three days in the grave. That is answered on a separate page : What does “he ascended up on high, he led captivity captive” mean? (Ephesians 4:8)
Contexts of Psalm 68 and Ephesians 4 compared.
Psalm 68 is a recounting of the triumphs of God for his people Israel throughout history. Although it is entitled ‘A Psalm of David’ this probably should be taken as a tribute to David, not least since “I will bring them back from Bashan” appears to reference the fall of the Northern Kingdom in 722 BCE and captives being taken to Syria. Which makes the psalm Hezekiah era or later. The most important event of rescue of captives however is the exodus from Egypt and that is the main geographical reference of Sinai in verses 17-18.
Ephesians 4 is unrelated to Old Testament history, it is talking about the rescue of captives from sin and death. This rescue starts with “one Lord (Jesus), one faith, one baptism” and then credits God the Father who is over all, including the Lord Jesus. So this is hardly a Trinitarian verse. It is also not a verse of much use to Arians and Oneness since it clearly shows the New Testament body as being in comparison with Old Testament Israel, two not suggesting that Jesus before his birth in any way existed to help Israel out of Egypt, or to rescue the captives of the Northern Kingdom from Bashan.
This is one of the simpler questions on verses where the work or characteristics of God in the Old Testament are cited in explaining the work of Chris in the new creation. Ephesians 4:8 is making a straightforward comparison from Psalm 68:18 “ascended on high” the release of captives from Egypt and the entry to the Sinai covenant God to Christ’s release of captives from death and sin and entry to the New Covenant. It is not claiming that Christ was present in Exodus.