Final Answer:
Sarah has finished reading 4 books.
Step-by-step explanation:
Sarah started the summer having read 0 books. As she has already finished reading 4 books and is currently reading her fifth summer book, the number of books she has completed so far is 4. When considering her current book as the one she's "reading" and not yet finished, the total number of finished books remains at 4.
Sarah's progress through her summer reading list shows she's completed 4 books. She began her summer reading with none finished and has steadily progressed through each book. Her current engagement with the fifth book indicates it's still in progress, leaving the count of completed books at 4. This interpretation aligns with the understanding that she's "currently reading" her fifth book and hasn't concluded it yet, hence the count of finished books remains unchanged at 4.