As-Sadooq explains what ‘Arsh is.

Shaykh Abu Ja'far As Sadooq says: 

Our belief concerning the Throne ('arsh) is that it is something which is carried or supported by the whole of creation. And 'arsh according to another interpretation is knowledge (ilm). And as-Sadiq, on whom be peace, was asked (the meaning of) the saying of Allah, the Mighty and Glorious:

“The Beneficent One, Who it established on the Throne” (Qur'an 20:5).

He said: He is equidistant from everything, and not a single thing is nearer to Him than another. Now that 'arsh, which is supported by the whole of creation,its borne by eight angels, each possessing eight eyes, each eye as large as the world.

One of the angels is of human shape and he asks Allah to provide daily bread for the sons of Adam. The second is of the shape of a bull, and he asks Allah to provide daily bread for all beasts. And the third is of the shape of a lion, and he asks Allah to provide daily bread for all beasts of prey. And the fourth is of the shape of a fowl, and he asks Allah to provide sustenance for all birds.

Today there exist four angels, but when the Day of Resurrection comes, they will become eight in number.

The 'arsh which means knowledge is borne by four amongst the ancients and four amongst the later ones; the former ones are Noah, Abraham, Moses and Jesus, on all of whom be peace; and the later are Muhammad, 'Ali, Hasan, and Husayn, the blessings of Allah upon them. This is what has been handed down from the Imams by a reliable chain of authorities concerning the Throne and its bearers.

Now the reason why these persons became the bearers of the 'arsh, that is the knowledge (of Allah), is that the ancient prophets, who lived prior to our Prophet Muhammad, namely, Noah, Abraham, Moses and Jesus, brought four different laws. It was through these that the true knowledge passed to them (i.e. Muhammad, Ali, etc.). Similarly the true knowledge was transmitted after Muhammad, by 'Ali, Hasan  and Husayn to those amongst the Imams who came after Husayn.

📚Al-Iqtiqadat P45-46