What are the four levels of protein structure?

Primary - The sequence of amino acids in the Protein, defined by the DNA sequence coding for the protein. 

Secondary - Any folding or coiling within the protein which are stabilised by hydrogen bonds. The two main types are alpha helices and beta pleated sheets.

Tertiary - The final three dimensional structure of a single polypeptide/protein which is determined by non covalent interactions, such as di-sulphide bridges, ionic bonds, hydrophobic/philic attractions and others. 

Quarternary - the joining of multiple polypeptides in a protein with more than one polypeptide chain, such as haemoglobin. Not all proteins have a quarternary structure.