By definition, the secondary structure of a protein is the hydrogen bonding between the amine and carbonyl groups in the amino acid chain. This usually occurs in the form of alpha-helices or beta-pleated sheets.

The linear sequence of the amino acids formed by peptide bonds is the primary protein structure. Interactions of R groups determines the tertiary structure. These interactions can be in the form of disulfide bonds, hydrogen bonding, or hydrophobic interactions.

This question is primarily asking the difference between quaternary protein structure and lower levels of folding. In quaternary protein structure two or more folded polypeptide chains interact with one another to form a functional protein. In the case of hemoglobin, we are told that there are four subunits, indicating that there are four polypeptide chains. It does not matter that there are two copies of each subunit; they are each their own polypeptide chain. Subunits are folded independently first, then joined into quaternary structure by non-covalent intermolecular forces.

Secondary protein structure is exclusively dependent on hydrogen bonding.

Primary protein structure is established by the sequence of amino acid residues, joined by covalent peptide bonds at the ribosome. Once the primary structure is established, secondary structure arises as a result of hydrogen bonding between the backbones of the amino acids (not the functional groups). Secondary structures take the forms of alpha-helices or beta-pleated sheets, both of which can exist within a single molecule. Tertiary structure forms from hydrogen bonding between functional groups, hydrophobic interactions, and disulfide linkages. Quaternary structure can involve hydrogen bonding and other intermolecular forces and is present only when multiple polypeptides come together to form a single protein complex.

Amino acids are covalently linked to one another by peptide bonds. The carboxylic acid portion of an amino acid connects to the amino terminus of the other amino acid, producing water as a byproduct. Peptide linkages are formed in the ribosome complex and result in the primary structure of the protein, Later, hydrogen bonding between amino acids plays a key role in secondary and tertiary protein structure.

Glycosidic bonds link adjacent monosaccharides in a carbohydrate polymer. Phosphodiester bonds are catalyzed by DNA ligase and are used to join nucleotides together to build nucleic acid chains.

At neutral pH an amino acid monomer will be found in the zwitterionic form in which there is a positive charge on the amino group and a negative charge on the carboxyl group. At very low an/or very high pH (less than 2 or greater than 12) there can be an overall negative or positive charge found on the amino acid, depending on the R-group.

Ionic bonds, disulfide bridges, hydrogen bonds and hydrophobic interactions are all examples of protein tertiary structure when they occur within a single polypeptide chain. However, if these interactions were to occur between separate polypeptide chains then they would be defining the quaternary structure of the protein. The linear sequence of amino acids within a protein makes up the primary structure. Protein secondary structure is defined by the localized three-dimensional structure of amino acids. These localized structures are normally constructed through hydrogen bonding networks. Alpha helices and Beta pleated sheets are examples of secondary structures. 

The linear sequence of amino acids within a protein makes up the primary structure.  Protein secondary structure is defined by the localized three-dimensional structure of of amino acids. These localized structures are normally constructed through hydrogen bonding networks. Alpha-helices and beta-pleated sheets are examples of secondary structures.  Protein tertiary structure is defined by the longer range interactions between amino acids within a single polypeptide chain. These interactions include ionic bonds, disulfide bridges, hydrogen bonds, and hydrophobic interactions. Protein quaternary structure is defined by the interactions between polypeptide chains. This often occurs in the formation of dimers and higher multimers.