Quaternary Structure
This is where proteins have more than one polypeptide chain held together in a precise 3D structure. They are held together with the same forces responsible for the formation of tertiary structures, these are: hydrogen, ionic, disulphide and van der waals forces.
They can also involve the addition of non-amino acid derived groups known as prosthetic groups. These can be formed from: metal ions, sugars, vitamins, methyl groups, phosphate groups etc.
An example of a quaternary protein is haemoglobin.