Book 5 - Your English comes ALIVE!
Now that you have finished Book 4, you are ready to practice, practice practice using all tenses. For many this book is "difficult" - the reason being you want to understand everything... and there is too much! Your learning will shift to trusting the method more. It will all become easier and easier with repetition. Many of the sentences are too complex to be able "translate" - so you have to jump into (if you haven't already) ONLY using English to learn English.