Was the Ofan on the land?!
Rashi: He was on the ground of the Raki'a.
Rashi citing Chagigah 13b: This is [the angel] Sandalfon - it stands on the ground, and its head reaches between the Chayos.
Radak: It was below, in a low place, like "Rukamti b'Sachtiyos Aretz" (Tehilim 139:15).
Why does it say "l'Arba'as Panav"?
Rashi #1: It is to the four faces of each Chayah.
Rashi #2: The Ofan has four faces.
What is the significance of the different kinds of angels that he saw?
Malbim: The Chayos conduct the Merkavah. The Ofanim are its wheels; the Chayos pull them. For each Chayah there is one Ofan for the four faces of the Chayah. Even though each Chayah has four faces, there is only one Ofan for each Chayah, four Ofanim for the four Chayos. Early Chachamim said that the encompassing Galgal 1 has four parts, and also the Galgal of the Mazalos; there are three Mazalos in each quarter. This divides the year into four Tekufos. The [phases of the] moon are divided into four stages in a month. The day is divided into four quarters. There are four directions in the world. Yechezkel saw in the four Chayos, four Sarim of kingdoms that will rise. Each moves one star that rules over and influences that nation. Each nation has a special star.
Malbim (4): Olam ha'Chayos is below Olam ha'Kisei, and above Olam ha'Ofanim, in which are the Galgalim (spheres on which the Heavenly bodies are fixed). The Chayos receive influence and power from Kisei Malchus Hashem, and move the Ofanim below. Therefore, first he tells how he saw Chayos, and then how they move the Ofanim, and then how they receive from the Kisei above them.
Malbim: Even though recent astronomers clarified that there is no sphere on which Heavenly bodies are fixed, their orbits are fixed on spheres, and they never depart from them.
