A protein with multiple identical subunits does indeed have a quaternary structure; in these cases, dimers and tetramers are common. The main forces holding together oligomeric subunits are weak, non-covalent interactions, specifically, hydrophobic ones, as well as electrostatic forces. Subunits do not necessarily form separate domains within a protein; in a potassium channel protein, for example, there are identical subunits which come together to form the single channel. Proteins’ 3D-structures do indeed sometimes change when ligands bind; this change help regulate the proteins’ biological activity.

Quaternary structure of a protein involves the assembly of subunits. Hemoglobin, p53 and DNA polymerase are all composed of subunits, while myoglobin is a functional single sequence. Since myoglobin does not have multiple subunits, it does not have quaternary structure.

Quaternary structure describes how polypeptide chains fit together to form a complete protein. Quaternary protein structure is held together by hydrophobic interactions, and disulfide bridges. The sequence of amino acids is known as primary structure; helices, sheets, and similar features are part of the secondary structure; and the 3-D organization is tertiary structure. "The four parts of a protein's amino acid sequence" does not refer to anything in particular.

Primary structure: linear sequence of amino acids

Secondary structure: hydrogen bonds, alpha-helices and beta-pleated sheets

Tertiary structure: disulfide bonds, single polypeptide chain

Myoglobin is a monomer, and is made of a single polypeptide chain. Thus, its highest level of protein structure is tertiary. While collagen does contain different polypeptide chains, it is an example of a protein with quaternary structure, not an explanation of what this means.

In this question, we're asked about how disulfide bonds relate to protein folding. Let's go through each form of structure.

Primary structure refers to the sequence of amino acids in the polypeptide, from the N-terminal end to the C-terminal end.

Secondary structure refers to local conformations of protein folding. There are a number of commonly found motifs that have been recognized, such as alpha-helices and beta-pleated sheets. These motifs are stabilized by intermolecular interactions between amino acid side-chains and also between alpha-carboxy and alpha-amino groups of the peptide backbone. Some of these intermolecular interactions include hydrogen bonding, van der Waals interactions, dipole interactions, and ionic bonding.

Tertiary structure refers to the overall three-dimensional structure of the folded polypeptide. This form of structure relies on the same intermolecular interactions found in secondary structure. In addition, tertiary structure also includes disulfide bonds that are found between cysteine residues.

Quaternary structure refers only to proteins that are composed of multiple polypeptides. These separate polypeptides are held together by the same intermolecular forces found in secondary and tertiary structures. In addition, disulfide bonds are also found in quaternary structure, just like in tertiary structure.

Thus, tertiary and quaternary structure both include disulfide bonds.

While covalent bonds create the primary structure of a protein, and hydrogen bonding and Van der Waals forces have a large impact on the secondary structure of a protein, they are not the main contributors to overall folding of a protein. This has more to do with solvation costs, hydrophobicity, and entropy. The hydrophobicity and hydrophobic portions of the protein must fold to minimize entropic costs.

Hemoglobin is a classic example of protein with a quaternary structure. The binding of oxygen to one sub unit increases the affinity of the other sub units for oxygen (cooperativity). Adult hemoglobin is made of two alpha globin and two beta globin polypeptides. Protein quaternary structure may involve both noncovalent and covalent forces. 

Proteins do not have maximum entropy in their native state. Folding requires order which decreases entropy in the system. The energy toll needed for this decrease in entropy is more than made up with by the increase in bonds formed when the protein folds into its native state. Therefore, the native state of a protein has the lowest energy. 

Protein folding diseases usually occur when beta-sheets alpha-helices misfold and precipitate into alpha-helices beta-sheets. This can lead to aggregation of amyloid deposits in the brain and neuronal apoptosis. Creutzfeld-Jacob Disease (CJD) and bovine spongiform encephalopathy (BSE, or mad cow disease) are examples of protein folding diseases.

The correct answer is "their binding to unfolded proteins is a passive, energy-free process." All molecular chaperones work by repeatedly binding to and releasing hydrophobic segments of unfolded proteins. This process is not passive and requires energy from the hydrolysis of ATP, which is why all molecular chaperones are also ATPases. All chaperones are proteins and they range in size from monomers to large multisubunit proteins.