If a graph is 4-list-colorable, then it is easy to see that it has exponentially many 5-list colorings. This is the first sentence in [Exponentially many 5-list-colorings of planar graphs,JCTB,97 (2007) 571–583]

I do not understand why a graph must have exponentially many 5-list colorings when it is 4-choosable.

Could any one explain the sentence above. Thanks