Final answer:
Cobalt has a hexagonal closest packed (hcp) structure, not a cubic structure. In comparison, chromium and aluminum have body-centered cubic (bcc) and face-centered cubic (fcc) structures with coordination numbers of eight and twelve, respectively.
Step-by-step explanation:
The crystal structure of cobalt is hexagonal closest packed (hcp) rather than cubic. Also, cobalt doesn't crystallize with four equivalent metal atoms in a cubic unit cell as initially described in the question. However, for the purpose of understanding crystal structures, we can compare cobalt to other metals which have cubic structures.
Metals like chromium crystallize in a body-centered cubic (bcc) structure, which has atoms at all of the corners and a single atom in the center of the cube. This bcc structure has a coordination number of eight.
On the other hand, metals such as aluminum have a face-centered cubic (fcc) structure with atoms at all the corners and at the centers of each face, yielding a coordination number of twelve. In the hcp structure that cobalt adopts, each cobalt atom also has a coordination number of twelve, similar to the fcc structure.