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It is not a 4 digit number. It shows you the significant, non-zero integer version of it. When you go over 9999, it will show 10000. Then 100000. And so on.
The full length of an integer in SQLite (the database engine used by Calibre) is enormous. Maximum value: 2^63-1 = 9223372036854775807
Once that value is reached, you will have to create a new Library and copy all of your books from the old Library to that new Library, wherein all of the newly added books will be assigned a new book id starting with 1.
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