The answer to your question would be that the best way to revise the selected sentence using the dash is the following one: I am learning about Japan believe it or not - it is a chain of around 4,000 islands.
The selected sentence is an example of the em dash. The em dash can be used in place of commas, parentheses, or colons. In this case, the em dash has taken the place of a colon. You can do this when you want to emphasize the conclusion of your sentence. However, the dash is less formal than the colon.
