The highest level of protein structure represented by haemoglobin is the quaternary protein structure.
This is because haemoglobin is composed of two polypeptide chains.
Brief summary of the protein structures:
PRIMARY: Amino acid sequence
SECONDARY: Folding of polypeptide into either an a-helix shape or b-pleated sheet shape
TERTIARY: Further folding of the polypeptide into a complex 3-D structure
QUATERNARY: Combination of two or more polypeptide chains
*From which the description of haemoglobin falls under the quaternary protein structure.
