What kind of text book for N4

Hi all, I just got the result from Nat-test that I passed the N5. So now I want to continue learning N4. What kind of text book or learning material that I should use? If anyone has any idea, please let me know. Thanks everyone.

P/S: the book, if possible, should be easy for self learning. So if you know that kind of book, please let me know.

by Comfortable-Ad9912

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