"This is the fourth book that I am reading today."
Translation:זה הספר הרביעי שאני קוראת היום.
They are the same in this context and most others. The only difference I think is that זה can be used to mean something more general like "things" or "the situation", whereas זהו always refers to something specific and grammatically singular and masculine, with the feminine version being זוהי and the plural אלו. But זה can be used in the same way, as in this sentence.