Can you explain the four levels of protein folding?

Proteins form from basic structures and fold into larger, more complex ones. They start as simple chains of amino acids, this is known as the primary structure of a protein. They then fold into secondary structures and form hydrogen bonds between the backbones of the peptide amino chain; they can either form structures known beta pleated sheets or alpha helixes. Further folding and formation of disulphide, hydrogen bonds, ionic bonds and hydrophobic interactions creates the tertiary structured protein which is a 3D subunit. Combinations of these 3D subunits then forms our final protein, the quaternary protein which has specific shape . An example of a quaternary protein is an enzyme.