Without checking that website, I believe the fourth type of balance is Approximate Balance. It's where minor differences in color, shape, and form create a nearly mirror image on either side of an imaginary line.
Artistic balance can be found in many ways.
Some examples:
A large simple dark shape is balanced by a smaller complex shape of brighter color.
Large empty spaces can be balanced by areas of textural interest and detail.
A large heavy shape is balanced by floating smaller ones.
Even the colors, values and visual textures of the fabrics can change the balance for better or worse.
Balance is just one of the design principles that should be found in quilts. Movement is another big one.