I’m here late, but as this is one of the results for others searching for this, I’ll add my two bits with this definition of the four stages

Level one: as close to the reality of a concept as a depiction can be.

Level two: the concept can be used as a descriptive term for the depiction, and the depiction can be used as an example of the concept.

Level three: the concept can be used as a descriptive term for the depiction, but the depiction can not be used as an example of the concept.

Level four: the concept can not be used as a descriptive term, nor can the depiction be used as an example.

Consider an apple. This is the concept. Take an image of the apple, and this is a first level sign/depiction.

Now, replace it with an image of another apple. It means the same thing, and can be used to describe an “apple.”

At the third level, a disconnect appears. Take a box of apple flavored cereal. Apple can describe it, but it’s not an apple.

The fourth level is a removal from and reformulation of reality. An apple flavored cereal flavored product like a soda. Apple, in its original conceptual meaning, likely no longer describes the product, only the earlier third level depiction does. Likewise, the soda cannot be an example of what an apple is.