In his response to the joint Catholic-Orthodox declaration, Metropolitan Philaret of the ROCOR states that union with Rome is only possible if "they accept the Orthodox Faith as it is maintained until now in the Holy, Catholic, and Apostolic Church." 1
Absent from that final phrase is mention of the Mark of Oneness or Unity, which, of course, is a thing accepted by the Eastern Orthodox churches as they hold to the Nicene Creed (the 4 being mentioned, in addition to Orthodox, by Fr. Seraphim Cardoza here) which states:
[We Believe in] one holy, catholic, and apostolic Church 3
Of the Mark of Unity, the Catholic Encyclopedia says that:
The marks of the Church are certain unmistakeable signs, or distinctive characteristics which render the Church easily recognizable to all, and clearly distinguish it from every other religious society, especially from those which claim to be Christian in doctrine and origin.
[the Church which has Christ for its founder] must unite its members in unity of doctrine[...] in unity of worship[...] and in unity of government. 4
That's the Catholic Encyclopedia, though. I'm looking for Orthodox sources. The Wikipedia article doesn't really talk about the Orthodox perspective. The OrthodoxWiki does not have an article on either the Four Marks, or of Unity as a Mark. Is my phrasing the problem? Should I be using different words? What are some sources from Eastern Orthodoxy about the 4 Marks and/or specifically about the Mark of Unity.
As of 08-Mar-2022 I found that there is [this article][https://orthodoxwiki.org/Primacy_and_Unity_in_Orthodox_Ecclesiology] on OrthodoxWiki that has a couple sources to use. Namely Metropolitan John Zizioulas, Fr. John Meyendorff, Fr. Alexander Schmemann, and Fr. Thomas FitzGerald. But it seems I'll need to simply read their works for a non-wiki answer.