יחזקאל א:טו פירוש: רש״י

one wheel [was] on the ground On the floor of the expanse.

for its four faces of each living being, or the wheel had four faces. Our Rabbis said (Chag. 13b): “This is Sandalphon [the angel], who stands on the ground yet whose head reaches up among the living beings.” From the context we learn that the wheels were four.